SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549
                                  SCHEDULE 13D

                    Under the Securities Exchange Act of 1934
                               (Amendment No. 6)*

                                CABOT CORPORATION
                                (Name of Issuer)

                     COMMON STOCK, PAR VALUE $1.00 PER SHARE
                         (Title of Class of Securities)

                                    127055101
                                 (CUSIP Number)

                               SPO Advisory Corp.
                         591 Redwood Highway, Suite 3215
                          Mill Valley, California 94941
                                 (415) 383-6600

                                 with a copy to:

                                Alison S. Ressler
                             Sullivan & Cromwell LLP
                             1888 Century Park East
                       Los Angeles, California 90067-1725
                                 (310) 712-6600

           (Name, Address and Telephone Number of Person Authorized to
                       Receive Notices and Communications)

                                  June 15, 2006
             (Date of Event which Requires Filing of This Statement)

If the filing person has previously filed a statement on Schedule 13G to report
the acquisition that is the subject of this Schedule 13D, and is filing this
Schedule because of Rule 13d-1(e), 13d-1(f) or 13d-1(g), check the following
box |_|.

Note. Schedules filed in paper format shall include a signed original and five
copies of the schedule, including all exhibits. See Rule 13d-7 for other parties
to whom copies are to be sent.

*The remainder of this cover page shall be filled out for a reporting person's
initial filing on this form with respect to the subject class of securities, and
for any subsequent amendment containing information which would alter
disclosures provided in a prior cover page.

The information required on the remainder of this cover page shall not be deemed
to be "filed" for the purpose of Section 18 of the Securities Exchange Act of
1934 ("Act") or otherwise subject to the liabilities of that section of the Act
but shall be subject to all other provisions of the Act (however, see the
Notes).


                                                              Page 2 of 20 pages
CUSIP No. 127055101

--------------------------------------------------------------------------------
1.       NAME OF REPORTING PERSON
         SPO Partners II, L.P.
--------------------------------------------------------------------------------
2.       CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP:           (a)     |_|
                                                                     (b)     |X|
--------------------------------------------------------------------------------
3.       SEC Use Only
--------------------------------------------------------------------------------
4.       SOURCE OF FUNDS
         WC
--------------------------------------------------------------------------------
5.       CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO
         ITEMS 2(d) OR 2(e) |_|
--------------------------------------------------------------------------------
6.       CITIZENSHIP OR PLACE OF ORGANIZATION
         Delaware
--------------------------------------------------------------------------------
                          7.     SOLE VOTING POWER
                                 8,297,600 (1)
                          ------------------------------------------------------
Number Of Shares          8.     SHARED VOTING POWER
Beneficially Owned By            -0-
Each Reporting Person     ------------------------------------------------------
With                      9.     SOLE DISPOSITIVE POWER
                                 8,297,600 (1)
                          ------------------------------------------------------
                          10.    SHARED DISPOSITIVE POWER
                                 -0-
--------------------------------------------------------------------------------
11.      AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
         8,297,600 (1)
--------------------------------------------------------------------------------
12.      CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN
         SHARES |_|
--------------------------------------------------------------------------------
13.      PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
         13.08%
--------------------------------------------------------------------------------
14.      TYPE OF REPORTING PERSON
         PN
--------------------------------------------------------------------------------

(1) Power is exercised through its sole general partner, SPO Advisory Partners,
    L.P.


                                                              Page 3 of 20 pages
CUSIP No. 127055101

--------------------------------------------------------------------------------
1.       NAME OF REPORTING PERSON
         SPO Advisory Partners, L.P.
--------------------------------------------------------------------------------
2.       CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP:           (a)     |_|
                                                                     (b)     |X|
--------------------------------------------------------------------------------
3.       SEC Use Only
--------------------------------------------------------------------------------
4.       SOURCE OF FUNDS
         Not Applicable
--------------------------------------------------------------------------------
5.       CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO
         ITEMS 2(d) OR 2(e) |_|
--------------------------------------------------------------------------------
6.       CITIZENSHIP OR PLACE OF ORGANIZATION
         Delaware
--------------------------------------------------------------------------------
                          7.     SOLE VOTING POWER
                                 8,297,600 (1)(2)
                          ------------------------------------------------------
Number Of Shares          8.     SHARED VOTING POWER
Beneficially Owned By            -0-
Each Reporting Person     ------------------------------------------------------
With                      9.     SOLE DISPOSITIVE POWER
                                 8,297,600 (1)(2)
                          ------------------------------------------------------
                          10.    SHARED DISPOSITIVE POWER
                                 -0-
--------------------------------------------------------------------------------
11.      AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
         8,297,600 (1)(2)
--------------------------------------------------------------------------------
12.      CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN
         SHARES |_|
--------------------------------------------------------------------------------
13.      PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
         13.08%
--------------------------------------------------------------------------------
14.      TYPE OF REPORTING PERSON
         PN
--------------------------------------------------------------------------------

(1) Solely in its capacity as the sole general partner of SPO Partners II, L.P.
(2) Power is exercised through its sole general partner, SPO Advisory Corp.


                                                              Page 4 of 20 pages
CUSIP No. 127055101

--------------------------------------------------------------------------------
1.       NAME OF REPORTING PERSON
         San Francisco Partners II, L.P.
--------------------------------------------------------------------------------
2.       CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP:           (a)     |_|
                                                                     (b)     |X|
--------------------------------------------------------------------------------
3.       SEC Use Only
--------------------------------------------------------------------------------
4.       SOURCE OF FUNDS
         WC
--------------------------------------------------------------------------------
5.       CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO
         ITEMS 2(d) OR 2(e) |_|
--------------------------------------------------------------------------------
6.       CITIZENSHIP OR PLACE OF ORGANIZATION
         California
--------------------------------------------------------------------------------
                          7.     SOLE VOTING POWER
                                 309,400 (1)
                          ------------------------------------------------------
Number Of Shares          8.     SHARED VOTING POWER
Beneficially Owned By            -0-
Each Reporting Person     ------------------------------------------------------
With                      9.     SOLE DISPOSITIVE POWER
                                 309,400 (1)
                          ------------------------------------------------------
                          10.    SHARED DISPOSITIVE POWER
                                 -0-
--------------------------------------------------------------------------------
11.      AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
         309,400 (1)
--------------------------------------------------------------------------------
12.      CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN
         SHARES |_|
--------------------------------------------------------------------------------
13.      PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
         0.49%
--------------------------------------------------------------------------------
14.      TYPE OF REPORTING PERSON
         PN
--------------------------------------------------------------------------------

(1) Power is exercised through its sole general partner, SF Advisory Partners,
    L.P.


                                                              Page 5 of 20 pages
CUSIP No. 127055101

--------------------------------------------------------------------------------
1.       NAME OF REPORTING PERSON
         SF Advisory Partners, L.P.
--------------------------------------------------------------------------------
2.       CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP:           (a)     |_|
                                                                     (b)     |X|
--------------------------------------------------------------------------------
3.       SEC Use Only
--------------------------------------------------------------------------------
4.       SOURCE OF FUNDS
         Not Applicable
--------------------------------------------------------------------------------
5.       CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO
         ITEMS 2(d) OR 2(e) |_|
--------------------------------------------------------------------------------
6.       CITIZENSHIP OR PLACE OF ORGANIZATION
         Delaware
--------------------------------------------------------------------------------
                          7.     SOLE VOTING POWER
                                 309,400 (1)(2)
                          ------------------------------------------------------
Number Of Shares          8.     SHARED VOTING POWER
Beneficially Owned By            -0-
Each Reporting Person     ------------------------------------------------------
With                      9.     SOLE DISPOSITIVE POWER
                                 309,400 (1)(2)
                          ------------------------------------------------------
                          10.    SHARED DISPOSITIVE POWER
                                 -0-
--------------------------------------------------------------------------------
11.      AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
         309,400 (1)(2)
--------------------------------------------------------------------------------
12.      CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN
         SHARES |_|
--------------------------------------------------------------------------------
13.      PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
         0.49%
--------------------------------------------------------------------------------
14.      TYPE OF REPORTING PERSON
         PN
--------------------------------------------------------------------------------

(1) Solely in its capacity as the sole general partner of San Francisco
    Partners II, L.P.
(2) Power is exercised through its sole general partner, SPO Advisory Corp.


                                                              Page 6 of 20 pages
CUSIP No. 127055101

--------------------------------------------------------------------------------
1.       NAME OF REPORTING PERSON
         SPO Advisory Corp.
--------------------------------------------------------------------------------
2.       CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP:           (a)     |_|
                                                                     (b)     |X|
--------------------------------------------------------------------------------
3.       SEC Use Only
--------------------------------------------------------------------------------
4.       SOURCE OF FUNDS
         Not Applicable
--------------------------------------------------------------------------------
5.       CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO
         ITEMS 2(d) OR 2(e) |_|
--------------------------------------------------------------------------------
6.       CITIZENSHIP OR PLACE OF ORGANIZATION
         Delaware
--------------------------------------------------------------------------------
                          7.     SOLE VOTING POWER
                                 8,607,000 (1)(2)
                          ------------------------------------------------------
Number Of Shares          8.     SHARED VOTING POWER
Beneficially Owned By            -0-
Each Reporting Person     ------------------------------------------------------
With                      9.     SOLE DISPOSITIVE POWER
                                 8,607,000 (1)(2)
                          ------------------------------------------------------
                          10.    SHARED DISPOSITIVE POWER
                                 -0-
--------------------------------------------------------------------------------
11.      AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
         8,607,000 (1)(2)
--------------------------------------------------------------------------------
12.      CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN
         SHARES |_|
--------------------------------------------------------------------------------
13.      PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
         13.57%
--------------------------------------------------------------------------------
14.      TYPE OF REPORTING PERSON
         CO
--------------------------------------------------------------------------------

(1) Solely in its capacity as the sole general partner of SPO Advisory
    Partners, L.P. with respect to 8,297,600 of such shares; and
    solely in its capacity as the sole general partner of SF Advisory
    Partners, L.P. with respect to 309,400 of such shares.

(2) Power is exercised through its three controlling persons, John H. Scully,
    William E. Oberndorf and William J. Patterson.


                                                              Page 7 of 20 pages
CUSIP No. 127055101

--------------------------------------------------------------------------------
1.       NAME OF REPORTING PERSON
         John H. Scully
--------------------------------------------------------------------------------
2.       CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP:           (a)     |_|
                                                                     (b)     |X|
--------------------------------------------------------------------------------
3.       SEC Use Only
--------------------------------------------------------------------------------
4.       SOURCE OF FUNDS
         Not Applicable
--------------------------------------------------------------------------------
5.       CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO
         ITEMS 2(d) OR 2(e) |_|
--------------------------------------------------------------------------------
6.       CITIZENSHIP OR PLACE OF ORGANIZATION
         USA
--------------------------------------------------------------------------------
                          7.     SOLE VOTING POWER
                                 -0-
                          ------------------------------------------------------
Number Of Shares          8.     SHARED VOTING POWER
Beneficially Owned By            8,607,000 (1)
Each Reporting Person     ------------------------------------------------------
With                      9.     SOLE DISPOSITIVE POWER
                                 -0-
                          ------------------------------------------------------
                          10.    SHARED DISPOSITIVE POWER
                                 8,607,000 (1)
--------------------------------------------------------------------------------
11.      AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
         8,607,000 (1)
--------------------------------------------------------------------------------
12.      CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN
         SHARES |_|
--------------------------------------------------------------------------------
13.      PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
         13.57%
--------------------------------------------------------------------------------
14.      TYPE OF REPORTING PERSON
         IN
--------------------------------------------------------------------------------

(1) These shares may be deemed to be beneficially owned by Mr. Scully solely
    in his capacity as one of three controlling persons of SPO Advisory Corp.


                                                              Page 8 of 20 pages
CUSIP No. 127055101

--------------------------------------------------------------------------------
1.       NAME OF REPORTING PERSON
         William E. Oberndorf
--------------------------------------------------------------------------------
2.       CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP:           (a)     |_|
                                                                     (b)     |X|
--------------------------------------------------------------------------------
3.       SEC Use Only
--------------------------------------------------------------------------------
4.       SOURCE OF FUNDS
         Not Applicable
--------------------------------------------------------------------------------
5.       CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO
         ITEMS 2(d) OR 2(e) |_|
--------------------------------------------------------------------------------
6.       CITIZENSHIP OR PLACE OF ORGANIZATION
         USA
--------------------------------------------------------------------------------
                          7.     SOLE VOTING POWER
                                 -0-
                          ------------------------------------------------------
Number Of Shares          8.     SHARED VOTING POWER
Beneficially Owned By            9,050,650 (1)
Each Reporting Person     ------------------------------------------------------
With                      9.     SOLE DISPOSITIVE POWER
                                 -0-
                          ------------------------------------------------------
                          10.    SHARED DISPOSITIVE POWER
                                 9,050,650 (1)
--------------------------------------------------------------------------------
11.      AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
         9,050,650 (1)
--------------------------------------------------------------------------------
12.      CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN
         SHARES |_|
--------------------------------------------------------------------------------
13.      PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
         14.27%
--------------------------------------------------------------------------------
14.      TYPE OF REPORTING PERSON
         IN
--------------------------------------------------------------------------------

(1) Of these shares, 8,607,000 shares may be deemed to be beneficially owned by
Mr. Oberndorf solely in his capacity as one of the three controlling persons of
SPO Advisory Corp., 333,650 shares may be deemed to be beneficially owned by Mr.
Oberndorf solely in his capacity as a trustee of the William and Susan Oberndorf
Trust, dated 10/19/98, 100,000 shares may be deemed to be beneficially owned
by Mr. Oberndorf solely in his capacity as sole general partner of Oberndorf
Family Partners, a California limited partnership, and 10,000 shares are owned
by Mr. Oberndorf solely in his capacity as trustee for the account of his
children.


                                                              Page 9 of 20 pages
CUSIP No. 127055101

--------------------------------------------------------------------------------
1.       NAME OF REPORTING PERSON
         William and Susan Oberndorf Trust, dated 10/19/98
--------------------------------------------------------------------------------
2.       CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP:           (a)     |_|
                                                                     (b)     |X|
--------------------------------------------------------------------------------
3.       SEC Use Only
--------------------------------------------------------------------------------
4.       SOURCE OF FUNDS
         PF
--------------------------------------------------------------------------------
5.       CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO
         ITEM 2(d) OR 2(e) |_|
--------------------------------------------------------------------------------
6.       CITIZENSHIP OR PLACE OF ORGANIZATION
         California
--------------------------------------------------------------------------------
                          7.     SOLE VOTING POWER
                                 333,650 (1)
                          ------------------------------------------------------
Number Of Shares          8.     SHARED VOTING POWER
Beneficially Owned By            -0-
Each Reporting Person     ------------------------------------------------------
With                      9.     SOLE DISPOSITIVE POWER
                                 333,650 (1)
                          ------------------------------------------------------
                          10.    SHARED DISPOSITIVE POWER
                                 -0-
--------------------------------------------------------------------------------
11.      AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
         333,650 (1)
--------------------------------------------------------------------------------
12.      CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN
         SHARES |_|
--------------------------------------------------------------------------------
13.      PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
         0.53%
--------------------------------------------------------------------------------
14.      TYPE OF REPORTING PERSON
         OO
--------------------------------------------------------------------------------

(1) Power is exercised through its trustees, William E. Oberndorf and Susan C.
Oberndorf.


                                                             Page 10 of 20 pages
CUSIP No. 127055101

--------------------------------------------------------------------------------
1.       NAME OF REPORTING PERSON
         Oberndorf Family Partners
--------------------------------------------------------------------------------
2.       CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP:           (a)     |_|
                                                                     (b)     |X|
--------------------------------------------------------------------------------
3.       SEC Use Only
--------------------------------------------------------------------------------
4.       SOURCE OF FUNDS
         WC
--------------------------------------------------------------------------------
5.       CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO
         ITEMS 2(d) OR 2(e) |_|
--------------------------------------------------------------------------------
6.       CITIZENSHIP OR PLACE OF ORGANIZATION
         California
--------------------------------------------------------------------------------
                          7.     SOLE VOTING POWER
                                 100,000 (1)
                          ------------------------------------------------------
Number Of Shares          8.     SHARED VOTING POWER
Beneficially Owned By            -0-
Each Reporting Person     ------------------------------------------------------
With                      9.     SOLE DISPOSITIVE POWER
                                 100,000 (1)
                          ------------------------------------------------------
                          10.    SHARED DISPOSITIVE POWER
                                 -0-
--------------------------------------------------------------------------------
11.      AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
         100,000 (1)
--------------------------------------------------------------------------------
12.      CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN
         SHARES |_|
--------------------------------------------------------------------------------
13.      PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
         0.16%
--------------------------------------------------------------------------------
14.      TYPE OF REPORTING PERSON
         PN
--------------------------------------------------------------------------------

(1) Power is exercised through its sole general partner, William E. Oberndorf.


                                                             Page 11 of 20 pages
CUSIP No. 127055101

--------------------------------------------------------------------------------
1.       NAME OF REPORTING PERSON
         William J. Patterson
--------------------------------------------------------------------------------
2.       CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP:           (a)     |_|
                                                                     (b)     |X|
--------------------------------------------------------------------------------
3.       SEC Use Only
--------------------------------------------------------------------------------
4.       SOURCE OF FUNDS
         Not Applicable
--------------------------------------------------------------------------------
5.       CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO
         ITEMS 2(d) OR 2(e) |_|
--------------------------------------------------------------------------------
6.       CITIZENSHIP OR PLACE OF ORGANIZATION
         USA
--------------------------------------------------------------------------------
                          7.     SOLE VOTING POWER
                                 -0-
                          ------------------------------------------------------
Number Of Shares          8.     SHARED VOTING POWER
Beneficially Owned By            8,607,000 (1)
Each Reporting Person     ------------------------------------------------------
With                      9.     SOLE DISPOSITIVE POWER
                                 -0-
                          ------------------------------------------------------
                          10.    SHARED DISPOSITIVE POWER
                                 8,607,000 (1)
--------------------------------------------------------------------------------
11.      AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
         8,607,000 (1)
--------------------------------------------------------------------------------
12.      CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN
         SHARES |_|
--------------------------------------------------------------------------------
13.      PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
         13.57%
--------------------------------------------------------------------------------
14.      TYPE OF REPORTING PERSON
         IN
--------------------------------------------------------------------------------

(1) These shares may be deemed to be beneficially owned by Mr. Patterson solely
    in his capacity as one of three controlling persons of SPO Advisory Corp.


                                                             Page 12 of 20 pages

--------------------------------------------------------------------------------
1.       NAME OF REPORTING PERSON
         Betty Jane Weimer
--------------------------------------------------------------------------------
2.       CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP:           (a)     |_|
                                                                     (b)     |X|
--------------------------------------------------------------------------------
3.       SEC Use Only
--------------------------------------------------------------------------------
4.       SOURCE OF FUNDS
         PF
--------------------------------------------------------------------------------
5.       CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO
         ITEMS 2(d) OR 2(e) |_|
--------------------------------------------------------------------------------
6.       CITIZENSHIP OR PLACE OF ORGANIZATION
         California
--------------------------------------------------------------------------------
                          7.     SOLE VOTING POWER
                                 5,000
                          ------------------------------------------------------
Number Of Shares          8.     SHARED VOTING POWER
Beneficially Owned By            -0-
Each Reporting Person     ------------------------------------------------------
With                      9.     SOLE DISPOSITIVE POWER
                                 5,000
                          ------------------------------------------------------
                          10.    SHARED DISPOSITIVE POWER
                                 -0-
--------------------------------------------------------------------------------
11.      AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
         5,000
--------------------------------------------------------------------------------
12.      CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN
         SHARES |_|
--------------------------------------------------------------------------------
13.      PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
         0.01%
--------------------------------------------------------------------------------
14.      TYPE OF REPORTING PERSON
         IN
--------------------------------------------------------------------------------


                                                             Page 13 of 20 pages

         This Amendment No. 6 amends the Schedule 13D (the "Original 13D") filed
with the Securities and Exchange Commission ("SEC") on August 29, 2005 and as
amended on September 14, 2005, September 26, 2005, October 20, 2005, November 8,
2005 and January 12, 2006. Unless otherwise stated herein, the Original 13D, as
previously amended, remains in full force and effect. Terms used herein and not
defined herein shall have the meanings ascribed thereto in the Original 13D.

ITEM 2.  IDENTITY AND BACKGROUND

         Item 2(a) is hereby amended and restated in its entirety as follows:

         "The undersigned hereby file this Schedule 13D Statement on behalf of
SPO Partners II, L.P., a Delaware limited partnership ("SPO"), SPO Advisory
Partners, L.P., a Delaware limited partnership ("SPO Advisory Partners"), San
Francisco Partners II, L.P., a California limited partnership ("SFP"), SF
Advisory Partners, L.P., a Delaware limited partnership ("SF Advisory
Partners"), SPO Advisory Corp., a Delaware corporation ("SPO Advisory Corp."),
John H. Scully ("JHS"), William E. Oberndorf ("WEO"), William J. Patterson
("WJP"), the William and Susan Oberndorf Trust, dated 10/19/98 ("Oberndorf
Trust"), Oberndorf Family Partners, a California limited partnership ("OFP"),
and Betty Jane Weimer ("BJW"). SPO, SPO Advisory Partners, SFP, SF Advisory
Partners, SPO Advisory Corp., JHS, WEO, WJP, the Oberndorf Trust, OFP and BJW
are sometimes hereinafter referred to as the "Reporting Persons." The Reporting
Persons are making this single, joint filing because they may be deemed to
constitute a "group" within the meaning of Section 13(d)(3) of the Securities
Exchange Act of 1934, as amended (the "Act"), although neither the fact of this
filing nor anything contained herein shall be deemed to be an admission by the
Reporting Persons that a group exists."

         The following subsections are hereby added to Item 2(b)-(c) in
appropriate alphabetical order as follows:

         "BJW

         "BJW's business address is 591 Redwood Highway, Suite 3215, Mill
Valley, California 94941. She is retired."


                                                             Page 14 of 20 pages

ITEM 3.  SOURCE AND AMOUNT OF FUNDS OR OTHER CONSIDERATION.

         Item 3 is hereby amended and restated in its entirety as follows:

         The source and amount of the funds used or to be used by the Reporting
Persons to purchase Shares are as follows:

NAME                        SOURCE OF FUNDS                  AMOUNT OF FUNDS
---------------------       ---------------------------      ---------------
SPO                         Contributions from Partners         $266,984,142
SPO Advisory Partners       Not Applicable                    Not Applicable
SFP                         Contributions from Partners          $10,038,218
SF Advisory Partners        Not Applicable                    Not Applicable
SPO Advisory Corp.          Not Applicable                    Not Applicable
JHS                         Not Applicable                    Not Applicable
WEO                         Personal Funds (1)                      $373,188
WJP                         Not Applicable                    Not Applicable
Oberndorf Trust             Personal Funds (1)                   $12,237,424
OFP                         Contribution from Partners           $ 3,635,448
BJW                         Personal Funds (1)                   $   186,594

----------
(1) As used herein, the term "Personal Funds" includes sums borrowed from banks
and brokerage firm margin accounts, none of which were borrowed or otherwise
obtained for the specified purpose of acquiring, holding, trading or voting
Shares.

ITEM 5.  INTEREST IN SECURITIES OF THE ISSUER.

         Item 5 is hereby amended and restated in its entirety as follows:

         (a) Percentage interest calculations for each Reporting Person are
based upon the Issuer having 63,421,612 total outstanding shares of Common Stock
as reported on the Issuer's 10-Q filed with the Securities and Exchange
Commission on May 10, 2006.

         SPO

         The aggregate number of Shares that SPO owns beneficially, pursuant to
Rule 13d-3 of the Act, is 8,297,600 Shares, which constitutes approximately
13.08% of the outstanding Shares.

         SPO Advisory Partners

         Because of its position as the sole general partner of SPO, SPO
Advisory Partners may, pursuant to Rule 13d-3 of the Act, be deemed to be the
beneficial owner of 8,297,600 Shares, which constitutes approximately 13.08%
of the outstanding Shares.



                                                             Page 15 of 20 pages

         SFP

         The aggregate number of Shares that SFP owns beneficially, pursuant to
Rule 13d-3 of the Act, is 309,400 Shares, which constitutes approximately
0.49% of the outstanding Shares.

         SF Advisory Partners

         Because of its position as the sole general partner of SFP, SF Advisory
Partners may, pursuant to Rule 13d-3 of the Act, be deemed to be the beneficial
owner of 309,400 Shares, which constitutes approximately 0.49% of the
outstanding Shares.

         SPO Advisory Corp.

         Because of its positions as the sole general partner of each of SPO
Advisory Partners and SF Advisory Partners, SPO Advisory Corp. may, pursuant to
Rule 13d-3 of the Act, be deemed to be the beneficial owner of 8,607,000
Shares in the aggregate, which constitutes approximately 13.57% of the
outstanding Shares.

         JHS

         Because of his position as a control person of SPO Advisory Corp., JHS
may, pursuant to Rule 13d-3 of the Act, be deemed to be the beneficial owner of
8,607,000 Shares, which constitutes approximately 13.57% of the outstanding
Shares.

         WEO

         Individually, and because of his positions as a control person of SPO
Advisory Corp., a trustee of the Oberndorf Trust and sole general partner of OFP
and as trustee for the account of his children, WEO may, pursuant to Rule 13d-3
of the Act, be deemed to be the beneficial owner of 9,050,650 Shares in the
aggregate, which constitutes approximately 14.27% of the outstanding Shares.

         WJP

         Because of his position as a control person of SPO Advisory Corp., WJP
may, pursuant to Rule 13d-3 of the Act, be deemed to be the beneficial owner of
8,607,000 Shares, which constitutes approximately 13.57% of the outstanding
Shares.

         Oberndorf Trust

         The aggregate number of Shares that the Oberndorf Trust owns
beneficially, pursuant to Rule 13d-3 of the Act, is 333,650 Shares, which
constitutes approximately 0.53% of the outstanding Shares.

         OFP

         The aggregate number of Shares that OFP owns beneficially, pursuant to
Rule 13d-3 of the Act, is 100,000 Shares, which constitutes approximately 0.16%
of the outstanding Shares.

         BJW

         The aggregate number of Shares that BJW owns beneficially, pursuant to
Rule 13d-3 of the Act, is 5,000 Shares, which constitutes approximately 0.01% of
the outstanding Shares.



                                                             Page 16 of 20 pages

         To the best of the knowledge of each of the Reporting Persons, other
than as set forth above, none of the persons named in Item 2 hereof is the
beneficial owner of any Shares.

         (b) SPO

         Acting through its sole general partner, SPO has the sole power to vote
or to direct the vote and to dispose or to direct the disposition of 8,297,600
Shares.

         SPO Advisory Partners

         Acting through its sole general partner and in its capacity as the sole
general partner of SPO, SPO Advisory Partners has the sole power to vote or to
direct the vote and to dispose or to direct the disposition of 8,297,600
Shares.

         SFP

         Acting through its sole general partner, SFP has the sole power to vote
or to direct the vote and to dispose or to direct the disposition of 309,400
Shares.

         SF Advisory Partners

         Acting through its sole general partner and in its capacity as the sole
general partner of SFP, SF Advisory Partners has the sole power to vote or to
direct the vote and to dispose or to direct the disposition of 309,400 Shares.

         SPO Advisory Corp.

         Acting through its controlling persons and in its capacities as the
sole general partner of each of SPO Advisory Partners and SF Advisory Partners,
SPO Advisory Corp. has the sole power to vote or to direct the vote and to
dispose or to direct the disposition of 8,607,000 Shares in the aggregate.

         JHS

         As one of three controlling persons of SPO Advisory Corp., which is the
sole general partner of each of SPO Advisory Partners and SF Advisory Partners,
JHS may be deemed to have shared power with WEO and WJP to vote or to direct the
vote and to dispose or to direct the disposition of 8,607,000 Shares held by
SPO and SFP in the aggregate.

         WEO

         As one of three controlling persons of SPO Advisory Corp., which is the
sole general partner of each of SPO Advisory Partners and SF Advisory Partners,
WEO may be deemed to have shared power with JHS and WJP to vote or to direct the
vote and to dispose or to direct the disposition of 8,607,000 Shares held by SPO
and SFP in the aggregate. WEO may be deemed to have shared power to vote or to
direct the vote and to dispose or to direct the disposition of 333,650 Shares
held in the Oberndorf Trust. Individually, and because of his position as the
sole general partner of OFP, WEO has the sole power to vote or to direct the
vote and to dispose or to direct the disposition of 100,000 Shares held by OFP.
Solely in his capacity as trustee for the account of his children, WEO has the
sole power to vote or to direct the vote and to dispose or to direct the
disposition of 10,000 shares owned by his children.

         WJP

         As one of three controlling persons of SPO Advisory Corp., which is the
sole general partner of each of SPO Advisory Partners and SF Advisory Partners,
WJP may be deemed to have shared power with JHS and WEO to vote or to direct the
vote and to dispose or to direct the disposition of 8,607,000 Shares held by
SPO and SFP in the aggregate.



                                                             Page 17 of 20 pages

         Oberndorf Trust

         Acting through its trustees, Oberndorf Trust has the sole power to vote
or to direct the vote and to dispose or to direct the disposition of 333,650
Shares in the aggregate.

         OFP

         OFP has the sole power to vote or to direct the vote and to dispose or
to direct the disposition of 100,000 Shares.


         BJW

         BJW has the sole power to vote or to direct the vote and to dispose or
to direct the disposition of 5,000 Shares.

         (c) During the past 60 days, the Reporting Persons purchased Shares in
open market transactions as set forth on Schedule I attached hereto.

         Except as set forth in this paragraph (c), to the best of the knowledge
of each of the Reporting Persons, none of the persons named in response to
paragraph (a) has effected any transactions in Shares during the past 60 days.

         (d) Each of the Reporting Persons affirms that no person other than
such Reporting Person has the right to receive or the power to direct the
receipt of distributions with respect to, or the proceeds from the sale of,
Shares owned by such Reporting Person.

         (e) Not applicable.

ITEM 7.  MATERIAL TO BE FILED AS EXHIBITS.

Item 7 is hereby amended and restated in its entirety as follows:

Exhibit A:   Agreement pursuant to Rule 13d-1(k)

Exhibit B:   Power of Attorney


                                                             Page 18 of 20 pages

                                    SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify
that the information set forth in this statement is true, complete and correct.


Dated June 19, 2006                      By:       /s/ Kim M. Silva
                                               --------------------------------
                                                   Kim M. Silva

                                           Attorney-in-Fact for:

                                           SPO PARTNERS II, L.P.(1)
                                           SPO ADVISORY PARTNERS, L.P.(1)
                                           SAN FRANCISCO PARTNERS II, L.P.(1)
                                           SF ADVISORY PARTNERS, L.P.(1)
                                           SPO ADVISORY CORP.(1)
                                           JOHN H. SCULLY(1)
                                           WILLIAM E. OBERNDORF(1)
                                           WILLIAM J. PATTERSON(1)
                                           WILLIAM AND SUSAN OBERNDORF TRUST,
                                           DATED 10/19/98 (1)
                                           OBERNDORF FAMILY PARTNERS (1)
                                           BETTY JANE WEIMER (2)

                                           (1) A Power of Attorney
                                           authorizing Kim M. Silva to
                                           act on behalf of this
                                           person or entity has been previously
                                           filed with the Securities and
                                           Exchange Commission.

                                           (2) A Power of Attorney authorizing
                                           Kim M. Silva to act on behalf of
                                           this person or entity is filed as
                                           Exhibit B.


                                                             Page 19 of 20 pages



                                               SCHEDULE I TO SCHEDULE 13D

                                  DATE OF                                           PRICE PER       WHERE/HOW TRANSACTION
REPORTING PERSON                TRANSACTION         TYPE         NUMBER OF SHARES   SHARE ($)              EFFECTED
---------------------           -----------         ----         ----------------   ---------       ---------------------
                                                                                      
SPO Partners II, L.P.           6/13/06             Buy                  100         31.67           Open Market/Broker

SPO Partners II, L.P.           6/13/06             Buy                2,100         31.68           Open Market/Broker

SPO Partners II, L.P.           6/13/06             Buy                3,500         31.68           Open Market/Broker

SPO Partners II, L.P.           6/13/06             Buy               40,600         31.69           Open Market/Broker

SPO Partners II, L.P.           6/13/06             Buy              107,500         31.70           Open Market/Broker

SPO Partners II, L.P.           6/14/06             Buy                  100         31.69           Open Market/Broker

SPO Partners II, L.P.           6/14/06             Buy               71,000         31.70           Open Market/Broker

SPO Partners II, L.P.           6/14/06             Buy                1,900         31.85           Open Market/Broker

SPO Partners II, L.P.           6/14/06             Buy                  100         31.86           Open Market/Broker

SPO Partners II, L.P.           6/14/06             Buy                  200         31.87           Open Market/Broker

SPO Partners II, L.P.           6/14/06             Buy                  500         31.88           Open Market/Broker

SPO Partners II, L.P.           6/14/06             Buy                  900         31.89           Open Market/Broker

SPO Partners II, L.P.           6/14/06             Buy               67,700         31.90           Open Market/Broker

SPO Partners II, L.P.           6/14/06             Buy                9,800         31.90           Open Market/Broker

SPO Partners II, L.P.           6/15/06             Buy              343,000         32.50           Open Market/Broker




                                                            Page 20 of 20 pages

                                  EXHIBIT INDEX

EXHIBIT     DOCUMENT DESCRIPTION                      PAGE NO.
-------     ------------------------------------      --------
A           Agreement Pursuant to Rule 13d-1(k)       1

B           Power of Attorney                         2