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                                                                    EXHIBIT 4(j)

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                                   MAPCO, INC.




                       WILLIAMS HOLDINGS OF DELAWARE, INC.




                                       AND




                       THE FIRST NATIONAL BANK OF CHICAGO,




                                     TRUSTEE



                        --------------------------------


                          THIRD SUPPLEMENTAL INDENTURE




                           DATED AS OF MARCH 31, 1998


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                     SUPPLEMENTING THE INDENTURE DATED AS OF
                          FEBRUARY 25, 1997, AS AMENDED




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                          THIRD SUPPLEMENTAL INDENTURE


         THIRD SUPPLEMENTAL INDENTURE (the "Third Supplemental Indenture"),
dated as of March 31, 1998, by and among MAPCO, Inc. ("MAPCO"), a Delaware
corporation, Williams Holdings of Delaware, Inc. ("WHD"), a Delaware
corporation, and The First National Bank of Chicago, a national banking
association, as Trustee (the "Trustee")

                                   WITNESSETH:

         WHEREAS, MAPCO and the Trustee have entered into an Indenture dated as
of February 25, 1997, as amended by a Supplemental Indenture No. 1 dated March
5, 1997, and a Supplemental Indenture No. 2 dated March 5, 1997 (the
"Indenture"), pursuant to which Indenture MAPCO has issued certain 7.25% Notes
due 2009 and 7.70% Debentures due 2027 (collectively, the "Notes"); and

         WHEREAS, pursuant to an Agreement and Plan of Merger dated as of
November 23, 1997 by and among MAPCO, The Williams Companies, Inc. ("Williams")
and TML Acquisition Corp., a wholly-owned subsidiary of Williams ("Sub"), Sub
has been merged into MAPCO; and

         WHEREAS, effective as of the date hereof, the stock of MAPCO has been
transferred to WHD, a wholly-owned subsidiary of Williams and all of the
outstanding capital stock of MAPCO Petroleum, Inc. and MAPCO Natural Gas
Liquids, Inc. have been sold by MAPCO to WHD;

         WHEREAS, MAPCO Petroleum, Inc., MAPCO Natural Gas Liquids, Inc.
together constitute MAPCO's properties and assets substantially as an entirety;
and

         WHEREAS, Section 8.1 of the Indenture permits the Trustee and MAPCO to
enter into indentures supplemental to evidence succession of another person to
MAPCO and the assumption by such successor of the covenants and obligations of
MAPCO under the Indenture.

         NOW, THEREFORE, for and in consideration of the premises and the mutual
covenants contained herein and in the Indenture and for other good and valuable
consideration, the receipt and sufficiency of which are hereby acknowledged,
MAPCO,WHD, and the Trustee hereby agree as follows:

         Section 1. Definitions. Capitalized terms which are used but not
defined herein shall have the meanings ascribed to such terms in the Indenture.




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         Section 2. Assumption of Certain Obligations.

               (a) WHD hereby expressly assumes (i) the due and punctual payment
of the principal of, premium, if any, on, interest on, and any additional
amounts payable under the Indenture in respect of, the Notes and (ii) the
performance of all of the covenants provided for in the Indenture to be
performed or observed by MAPCO.

               (b) MAPCO and the Trustee hereby acknowledge that WHD shall
succeed to, and be substituted for, and may exercise every right and power of,
MAPCO under the Indenture with the same effect as if WHD had been named therein.

         Section 3. Effect of Third Supplemental Indenture. From and after the
execution and delivery of this Third Supplemental Indenture, the Indenture shall
be deemed to be modified as herein provided, but except as modified hereby, the
Indenture shall continue in full force and effect. The Indenture as modified
hereby shall be read, taken and construed as one and the same instrument.

         Section 4. Notice. Any notice or communication by the Trustee to WHD is
duly given if in writing and delivered in person or by express mail service to
the address set forth below:

                              Williams Holdings of Delaware, Inc.
                              One Williams Center
                              Tulsa, Oklahoma 74172
                              Attention:  Treasurer

         Section 5. Governing Law. This Third Supplemental Indenture shall be
governed by and construed in accordance with the laws bf the State of New York
(regardless of the laws that might otherwise govern underapplicable principles
of conflicts of laws) as to all matters, including, without limitation, matters
of validity, construction, effect, performance and remedies.

         Section 6. Counterparts. This Third Supplemental Indenture may be
executed in any number of counterparts, each of which, when so executed.and
delivered, shall be an original, but such counterparts shall together constitute
but one and the same instrument.



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         IN WITNESS WHEREOF, each of MAPCO, WHD and the Trustee has caused this
Third Supplemental Indenture to be executed on its behalf by its duly authorized
officer and has caused its official seal to be impressed hereon and attested by
one of its duly authorized officers, all as of the day and year first above
written.

[SEAL]                                       MAPCO, INC.

Attest

/s/ SHAWNA L. GEHRES                         By: /s/ PHILLIP D. WRIGHT
--------------------------------                --------------------------------
Name:    Shawna L. Gehres                    Name:    Phillip D. Wright
Title:   Assistant Secretary                 Title:   Senior Vice President

[SEAL]                                       WILLIAMS HOLDINGS OF DELAWARE, INC.

Attest

/s/ DAVID M. HIGBEE                          By: /s/ JAMES G. IVEY
--------------------------------                --------------------------------
Name:    David M. Higbee                     Name:    James G. Ivey
Title:   Secretary                           Title:   Treasurer

[SEAL]                                       THE FIRST NATIONAL BANK OF CHICAGO,
                                               AS TRUSTEE
Attest

/s/ ANN LONGINO                              By: /s/ JOHN R. PRENDIVILLE
--------------------------------                --------------------------------
Name:    Ann Longino                         Name:    John R. Prendiville
Title:   Trust Officer                       Title:   Vice President


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