EXHIBIT 3.12

                       LIMITED LIABILITY COMPANY AGREEMENT
                                       OF
                       THUNDER BASIN COAL COMPANY, L.L.C.

                  THE UNDERSIGNED are executing this Limited Liability Company
Agreement (the "Agreement") for the purpose of forming a limited liability
company (the "Company") pursuant to the provisions of the Delaware Limited
Liability Company Act, 6 Del. C. Sections 18-101 ~ seq. (the "Act"), and do
hereby agree as follows:

                  1.       Name. The name of the Company shall be Thunder Basin
Coal Company, L.L.C., or such other name as the Members may from time to time
hereafter designate.

                  2.       Definitions. Capitalized terms not otherwise defined
herein shall have the meanings set forth therefor in Section 18-101 of the Act.

                  3.       Purpose. The Company is formed for the purpose of
engaging in any lawful business permitted by the Act or the laws of any
jurisdiction in which the Company may do business. The Company shall have the
power to engage in all activities and transactions which the Members deem
necessary or advisable in connection with the foregoing.

                  4.       Offices.

                           (a)      The principal place of business and office
of the Company shall be located at, and the Company's business shall be
conducted from, such place or places as the Members may designate from time to
time.

                           (b)      The registered office of the Company in the
State of Delaware shall be located at do The Corporation Trust Company,
Corporation Trust Center, 1209 Orange Street, Wilmington, Delaware 19801. The
name and address of the registered agent of the Company for service of process
on the Company in the State of Delaware shall be The Corporation Trust Company,
Corporation Trust Center, 1209 Orange

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Street, Wilmington, Delaware 19801. The Members may from time to time change the
registered agent or office by an amendment to the certificate of formation of
the Company.

                  5.       Members. The name and business or residence address
of each Member of the Company are set forth on Schedule A attached hereto. The
business and affairs of the Company shall be managed by a Board of Directors
selected, and subject to removal with or without cause, by the Members which
shall have all powers and rights necessary, appropriate or advisable to
effectuate and carry out the purposes and business of the Company. The Directors
will be deemed "Managers" within the meaning of the Act. Each Member and
Director is hereby designated as an authorized person, within the meaning of the
Act, to execute, deliver and file the certificate of formation of the Company
(and any amendments and/or restatements thereof); provided that, except to the
extent specifically referenced in this sentence, no Member shall have the
authority to bind or otherwise act for the Company. Except for the power to
select and remove members of the Board of Directors, the Members shall have no
power or authority with respect to the operations of the Company and shall only
have the specific rights and privileges set forth herein, in the Bylaws or as
provided by applicable law. In connection with the management of the business
and affairs of the Company, the Board of Directors and officers of the Company
shall execute, deliver and file any certificates (and any amendments and/or
restatements thereof) necessary for the Company to qualified, to do business in
a jurisdiction in which the Company may wish to conduct business. The execution
by one Director, or by one Member, as applicable, of any of the foregoing
certificates (and any amendments and/or restatements thereof) shall be
sufficient.

                  6.       Term. The term of the Company shall commence on the
date of filing of the certificate of

                                       -2-


formation of the Company in accordance with the Act and shall continue until the
Company is dissolved and its affairs are wound up in accordance with Section 13
of this Agreement and a certificate of cancellation is filed in accordance with
the Act.

                  7.       Action by Members. Any action to be taken by the
Members of the Company shall require the affirmative vote of Members holding a
majority of the Limited Liability Company Interests of the Company (except as
otherwise expressly provided herein).

                  8.       Capital Contributions. Members shall make capital
contributions to the Company in such amounts and at such times as they shall
mutually agree pro rata in accordance with profit sharing interests as set forth
in Schedule A hereof ("Profit Sharing Interests"), which amounts shall be set
forth in the books and records of the Company.

                  9.       Assignments of Company Interest.

                           a.       The Members may not sell, assign, pledge, or
otherwise transfer or encumber (collectively "transfer") less than all of their
Interest in the Company under any circumstances. Members may not transfer all of
their Interest in the Company without the Board of Directors consenting to the
proposed transfer by the Member of all of its Interest in the Company which
consent may be given or withheld at the sole discretion of the Board of
Directors. The transferee of all of the Interest of a Member shall be admitted
as the successor Member as provided in Section 9(b) hereof.

                           b.       In the event there is at any time a proposed
transfer of all of the Interest of a Member of the Company to which the Board of
Directors has consented as provided in Section 9(a) hereof, the transfer of such
Interest to the transferee thereof and the admission of such transferee as a
Member of the

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Company shall be deemed to occur simultaneously with the withdrawal of the
transferring Member with the effect that, in connection with such transfer,
there shall at all times be at least one Member of the Company.

                           c.       The Board of Directors shall amend Schedule
I hereto from time to time to reflect transfers made in accordance with, and as
permitted under, this Section 9. Any purported transfer in violation of this
Section 9 shall be null and void and shall not be recognized by the Company.

                  10.      Resignation. No Member shall have the right to resign
from the Company except with the consent of all of the Members and upon such
terms and conditions as may be specifically agreed upon between the resigning
Member and the remaining Members. The provisions hereof with respect to
distributions upon resignation are exclusive and no Member shall be entitled to
claim any further or different distribution upon resignation under Section
18-604 of the Act or otherwise.

                  11.      Allocations and Distributions. Distributions of cash
or other assets of the Company shall be made at such times and in such amounts
as the Members may determine. Distributions shall be made to (and profits and
losses of the Company shall be allocated among) Members pro rata in accordance
with each of their Profit Sharing Interests, or in such other manner and in such
amounts as all of the Members shall agree from time to time and which shall be
reflected in the books and records of the Company.

                  12.      Return of Capital. No Member has the right to receive
any distributions which include a return of all or any part of such Member's
capital contribution, provided that upon the dissolution and winding up of the
Company, the assets of the Company shall be distributed as provided in Section
18-804 of the Act.

                  13.      Dissolution. The Company shall be dissolved and its
affairs wound up upon the affirmative vote of the Members acting in accordance
with Section 7 of this Agreement.

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                  14.      Amendments. This Agreement may be amended only upon
the written consent of all of the Members.

                  15.      Miscellaneous. Neither the Members nor the Directors
shall have any liability for the debts, obligations or liabilities of the
Company except to the extent provided by the Act. This Agreement shall be
governed by, and construed under, the laws of the State of Delaware, without
regard to conflict of law rules.

                  16.      Board of Directors and Officers. The Board of
Directors may adopt by-laws and resolutions as are necessary and appropriate for
the regulation of the affairs and the conduct of the business of the Company,
and may employ and retain persons as may be necessary or appropriate for the
conduct of the Company's business, including employees and agents who may be
designated as, officers with titles, including, but not limited to, "chairman,"
"chief executive officer," "president," executive vice-president," "vice
president," "treasurer," "secretary," "managing director," "chief financial
officer," "assistant treasurer" and "assistant secretary."

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                  IN WITNESS WHEREOF, the undersigned have duly executed this
Agreement as of July 10, 1997.

                          ATLANTIC RICHFIELD COMPANY,
                          MEMBER

                                By: /s/ BRUCE G. WHITMORE
                                    -----------------------------------
                                Name: Bruce G. Whitmore
                                Title: Senior Vice President and General Counsel



                                   SCHEDULE A
                                     REVISED
                              (AS OF JUNE 1, 1998)

         Pursuant to the contribution on .June 1, 1998 of membership interests
by Atlantic Richfield Company to Arch Western Resources, LLC per Contribution
Agreement among Arch Coal, Inc., Arch Western Acquisition Corporation, Atlantic
Richfield Company. Delta Housing Inc. and Arch Western Resources, LLC dated
March 22, 1998.



Name and Address of Member                     Profit Sharing Interests
- --------------------------                     ------------------------
                                            
Arch Western Resources, LLC
CityPlace One, Suite 300
St. Louis, Missouri 63141                                 100%