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                                                                    EXHIBIT 4.16

                      SECOND AMENDMENT TO CREDIT AGREEMENT

         THIS SECOND AMENDMENT TO CREDIT AGREEMENT (this "Second Amendment"),
dated as of June 5, 2002, is entered into among ELCOR CORPORATION a Delaware
corporation (the "Borrower"), the lenders listed on the signature pages hereof
(the "Lenders"), BANK ONE, N.A., as Documentation Agent, FIRST UNION NATIONAL
BANK, as Syndication Agent, and BANK OF AMERICA, N.A., as Administrative Agent,
Swing Line Lender and L/C Issuer.

                                   BACKGROUND

         A.       The Borrower, the Lenders, the Documentation Agent, the
Syndication Agent, the Administrative Agent, the Swing Line lender and the L/C
Issuer are parties to that certain Credit Agreement, dated as of November 30,
2000, as amended by that certain First Amendment to Credit Agreement, dated as
of March 31, 2001 (said Credit Agreement as amended, the "Credit Agreement").
The terms defined in the Credit Agreement and not otherwise defined herein shall
be used herein as defined in the Credit Agreement.

         B.       The Borrower has requested an amendment to the Credit
Agreement and has notified the Administrative Agent of a proposed reduction in
the Aggregate Commitments.

         C.       The Lenders, the Documentation Agent, the Syndication Agent,
the Administrative Agent, the Swing Line Lender and the L/C Issuer hereby agree
to amend the Credit Agreement, subject to the terms and conditions set forth
herein.

         NOW, THEREFORE, in consideration of the covenants, conditions and
agreements hereafter set forth, and for other good and valuable consideration,
the receipt and adequacy of which are all hereby acknowledged, the parties
hereto covenant and agree as follows:

             1.   AMENDMENTS.

                           (a)      Section 1.01 of the Credit Agreement is
                             hereby amended by adding the defined term "Private
                             Placement Debt" thereto in proper alphabetical
                             order to read as follows:

                  "Private Placement Debt" means that certain Indebtedness of
         the Borrower in the aggregate principal amount of $120,000,000 issued
         on or about June 7, 2002.

                           (b)      Section 7.09 of the Credit Agreement is
                             hereby amended to read as follows:

                  7.09     BURDENSOME AGREEMENTS. Enter into any Contractual
         Obligation that limits the ability (a) of any Subsidiary to make
         Restricted Payments to the Borrower or to otherwise transfer property
         to the Borrower or (b) of the Borrower or any Subsidiary to

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         create, incur, assume or suffer to exist Liens on property of such
         Person, except in respect of clause (b) for such limitation under (i)
         the Private Placement Debt and (ii) any Swap Contract entered into with
         any Lender or any Affiliate of any Lender.

                           (c)      Schedule 2.01 to the Credit Agreement is
                             hereby amended to be in the form of Schedule 2.01
                             to this Second Amendment and the Commitment of each
                             Lender and the Aggregate Commitments are hereby
                             reduced and amended as set forth therein.

             2.   REPRESENTATIONS AND WARRANTIES TRUE; NO EVENT OF DEFAULT. By
                  its execution and delivery hereof, Company represents and
                  warrants that, as of the date hereof and after giving effect
                  to the waiver contemplated by the foregoing Section 2:

                           (a)      the representations and warranties contained
                             in the Credit Agreement and the other Loan
                             Documents are true and correct on and as of the
                             date hereof as made on and as of such date;

                           (b)      no event has occurred and is continuing
                             which constitutes a Default or an Event of Default;

                           (c)      the Borrower has full power and authority to
                             execute and deliver this Second Amendment, and this
                             Second Amendment has been duly executed and
                             delivered by the Borrower, and this Second
                             Amendment and the Credit Agreement, as amended
                             hereby, constitute the legal, valid and binding
                             obligations of the Borrower, enforceable in
                             accordance with their respective terms, except as
                             enforceability may be limited by applicable debtor
                             relief laws and by general principles of equity
                             (regardless of whether enforcement is sought in a
                             proceeding in equity or at law) and except as
                             rights to indemnity may be limited by federal or
                             state securities laws; and

                           (d)      no authorization, approval, consent, or
                             other action by, notice to, or filing with, any
                             governmental authority or other Person (including
                             the consent of the Board of Directors of the
                             Borrower) is required for the execution, delivery
                             or performance by Company of this Second Amendment.

             3.   CONDITIONS TO EFFECTIVENESS. This Second Amendment shall be
                  effective upon satisfaction or completion of the following:

                           (a)      the Administrative Agent shall have received
                             counterparts of this Second Amendment executed by
                             the Required Lenders;

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                           (b)      the Administrative Agent shall have received
                             counterparts of this Second Amendment executed by
                             the Borrower and acknowledged by each Guarantor;

                           (c)      the Private Placement Debt shall have been
                             issued; and

                           (d)      the Administrative Agent shall have
                             received, in form and substance satisfactory to the
                             Administrative Agent and its counsel, such other
                             documents, certificates and instruments as the
                             Administrative Agent shall require.

             4.   REFERENCE TO THE CREDIT AGREEMENT.

                           (a)      Upon the effectiveness of this Second
                             Amendment, each reference in the Credit Agreement
                             to "this Agreement", "hereunder", or words of like
                             import shall mean and be a reference to the Credit
                             Agreement, as affected and amended hereby.

                           (b)      The Credit Agreement, as amended by the
                             amendments referred to above, shall remain in full
                             force and effect and is hereby ratified and
                             confirmed.

             5.   COSTS, EXPENSES AND TAXES. The Borrower agrees to pay on
                  demand all costs and expenses of the Administrative Agent in
                  connection with the preparation, reproduction, execution and
                  delivery of this Second Amendment and the other instruments
                  and documents to be delivered hereunder (including the
                  reasonable fees and out-of-pocket expenses of counsel for the
                  Administrative Agent with respect thereto).

             6.   GUARANTOR'S ACKNOWLEDGMENT. By signing below, each Guarantor
                  (a) acknowledges, consents and agrees to the execution,
                  delivery and performance by the Borrower of this Second
                  Amendment, (b) acknowledges and agrees that its obligations in
                  respect of its Guaranty (i) are not released, diminished,
                  waived, modified, impaired or affected in any manner by this
                  Second Amendment or any of the provisions contemplated herein,
                  (c) ratifies and confirms its obligations under its Guaranty,
                  and (d) acknowledges and agrees that it has no claims or
                  offsets against, or defenses or counterclaims to, its
                  Guaranty.

             7.   EXECUTION IN COUNTERPARTS. This Second Amendment may be
                  executed in any number of counterparts and by different
                  parties hereto in separate counterparts, each of which when so
                  executed and delivered shall be deemed to be an original and
                  all of which when taken together shall constitute but one and
                  the same instrument.

             8.   GOVERNING LAW; BINDING EFFECT. This Second Amendment shall be
                  governed by and construed in accordance with the laws of the
                  State of Texas

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                  applicable to agreements made and to be performed entirely
                  within such state, provided that each party shall retain all
                  rights arising under federal law, and shall be binding upon
                  the parties hereto and their respective successors and
                  assigns.

             9.   HEADINGS. Section headings in this Second Amendment are
                  included herein for convenience of reference only and shall
                  not constitute a part of this Second Amendment for any other
                  purpose.

         ENTIRE AGREEMENT. THE CREDIT AGREEMENT, AS AMENDED BY THIS SECOND
AMENDMENT, AND THE OTHER LOAN DOCUMENTS REPRESENT THE FINAL AGREEMENT BETWEEN
THE PARTIES AND MAY NOT BE CONTRADICTED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS,
OR SUBSEQUENT ORAL AGREEMENTS BETWEEN THE PARTIES. THERE ARE NO UNWRITTEN ORAL
AGREEMENTS BETWEEN THE PARTIES.

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         IN WITNESS WHEREOF, this Second Amendment is executed as of the date
first set forth above.

                                       ELCOR CORPORATION

                                       By: _____________________________________
                                            Name:  _____________________________
                                            Title: _____________________________

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                                       BANK OF AMERICA, N.A., as Administrative
                                       Agent

                                       By: _____________________________________
                                            Name:  _____________________________
                                            Title: _____________________________

                                       BANK OF AMERICA, N.A., as a Lender, L/C
                                       Issuer and Swing Line Lender

                                       By: _____________________________________
                                            Name:  _____________________________
                                            Title: _____________________________

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                                       BANK ONE, N.A., as a Lender and
                                       Documentation Agent

                                       By: _____________________________________
                                            Name:  _____________________________
                                            Title: _____________________________

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                                       FIRST UNION NATIONAL BANK, N.A., as a
                                       Lender and Syndication Agent

                                       By: _____________________________________
                                            Name:  _____________________________
                                            Title: _____________________________

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                                       THE BANK OF TOKYO-MITSUBISHI, LTD.,
                                       as a Lender

                                       By: _____________________________________
                                            Name:  _____________________________
                                            Title: _____________________________

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                                       THE FROST NATIONAL BANK, as a Lender

                                       By: _____________________________________
                                            Name:  _____________________________
                                            Title: _____________________________

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                                       COMERICA BANK-TEXAS, as a Lender

                                       By: _____________________________________
                                            Name:  _____________________________
                                            Title: _____________________________

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                                       COMPASS BANK, as a Lender

                                       By: _____________________________________
                                            Name:  _____________________________
                                            Title: _____________________________

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ACKNOWLEDGED AND AGREED TO:

ELK CORPORATION OF DALLAS
ELK CORPORATION OF TEXAS
ELK CORPORATION OF AMERICA
ELK CORPORATION OF ARKANSAS
ELK CORPORATION OF ALABAMA

By: ___________________________
    Name:______________________
    Title:_____________________

OEL, LTD.
CHROMIUM CORPORATION

By: ___________________________
    Name:______________________
    Title:_____________________

NELPA, INC.

By: ___________________________
    Name:______________________
    Title:_____________________

ELCOR SERVICE LIMITED PARTNERSHIP

By:      ELCOR MANAGEMENT CORPORATION,
         Its General Partner

By: ___________________________
    Name:______________________
    Title:_____________________

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ELCOR MANAGEMENT CORPORATION

By: ___________________________
    Name:______________________
    Title:_____________________

CYBERSHIELD OF GEORGIA, INC.
CYBERSHIELD, INC.
CYBERSHIELD INTERNATIONAL, INC.
CYBERSHIELD OF TEXAS, INC.
(formerly known as Chromium Corporation)

By: ___________________________
    Name:______________________
    Title:_____________________

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                                                                   SCHEDULE 2.01

                                   COMMITMENTS
                               AND PRO RATA SHARES

<TABLE>
<CAPTION>
           LENDER                            COMMITMENT           PRO RATA SHARE
           ------                            ----------           --------------
<S>                                      <C>                     <C>
Bank of America, N.A.                    $  22,857,142.86         22.857142857%

First Union National Bank, N.A.          $  18,571,428.57         18.571428571%

Bank One, N.A.                           $  18,571,428.57         18.571428571%

Comerica Bank - Texas                    $  14,285,714.29         14.285714286%

The Frost National Bank                  $  11,428,571.43         11.428571429%

The Bank of Tokyo-Mitsubishi, Ltd.       $   8,571,428.57          8.571428572%

Compass Bank                             $   5,714,285.71          5.714285714%

            TOTAL                        $1,00,000,000.00        100.000000000%
</TABLE>