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                                                                  EXHIBIT 10 (t)

                       FOURTH AMENDMENT TO LOAN AGREEMENT
                       ----------------------------------


  THIS FOURTH AMENDMENT ("Fourth Amendment") dated as of the 15th day of July,
1999, to the Loan Agreement (the "Loan Agreement"), made and entered into as of
July 12, 1995, by and among SUMMIT BANCSHARES, INC., a Texas corporation,
(hereinafter called "Borrower") and THE FROST NATIONAL BANK, a national banking
association (hereinafter called "Lender").  All capitalized terms not otherwise
defined herein shall have the meaning ascribed to each of them in the Loan
Agreement.

                                  WITNESSETH:

     WHEREAS, Borrower executed the Loan Agreement to govern those two certain
promissory notes from Lender, specifically a $8,000,000.00 Acquisition Note and
a $1,000,000.00 Liquidity Note (collectively, the "Notes").

     WHEREAS, Borrower executed the First Amendment to Loan Agreement on July
15, 1996 ("First Amendment") which renewed and extended the maturity date of the
Notes: and

     WHEREAS, Borrower executed the Second Amendment to Loan Agreement on July
15, 1997 ("Second Amendment") which renewed and extended the maturity date of
the Notes; and

     WHEREAS, Borrower executed the Third Amendment to Loan Agreement on July
15, 1998 ("Third Amendment") which renewed and extended the maturity date of the
Notes; and

     WHEREAS, the Borrower desires to again renew and extend the unpaid
principal balance of the Notes; and

     WHEREAS, the Lender agrees to renew and extend the Notes all as hereinafter
provided.

     NOW, THEREFORE, in consideration of the premises and other good and
valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, Borrower and Lender do hereby agree as follows:

                                   ARTICLE I
                                   ---------

                          Amendment to Loan Agreement
                          ---------------------------

     1.1  Amendment to Notes.  As of the effective date hereof, the Borrower has
          -------------------
zero outstanding under the Acquisition Note and the Liquidity Note.  The
Borrower desires to renew these credit facilities by the execution of another
Acquisition Note and Liquidity Note extending the original payment terms and the
maturity date by one year.  Accordingly, Sections 2.02 (a) and (b) of the Loan
Agreement shall be, and are hereby, amended to read in their entirety as
follows:

          (a)  Acquisition Note.  From Closing Date and continuing at all times
               -----------------
     through July 15, 2000 ( the "Revolving Credit Period") the Loan evidenced
     by the Acquisition Note shall be a revolving credit facility which will
     allow the Borrower to request such amounts as Borrower may elect from time
     to time (each such amount being herein called an "Advance") so long as the
     aggregate amount of Advances outstanding at any time under the Acquisition
     Note does not exceed Eight
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     Million and No/100 Dollars ($8,000,000.00) provided however, the minimum
     Advance must be at least $500,000.00. The Borrower shall have the right to
     borrow, repay, and borrow again under the credit facility. The outstanding
     principal balance of the Acquisition Note n July 15, 2000 shall convert to
     a term facility (the "Term Period") and shall be payable in 20 equal
     quarterly installments of principal plus all accrued and unpaid interest,
     with all unpaid principal plus all accrued and unpaid interest being due
     and payable on July 15, 2005. Principal and interest of the Acquisition
     Note shall be due and payable as provided in the Acquisition Note.

          (b)  Liquidity Note.  The Liquidity Note shall be due and payable as
               ---------------
     follows: during the Revolving Credit Period the Loan evidenced by the
     Liquidity Note shall be a revolving credit facility which will allow the
     Borrower to request such amounts as Borrower may elect from time to time
     (each such amount being herein called an "Advance") so long as the
     aggregate amount of Advances outstanding at any time under the Liquidity
     Note does not exceed One Million and No/100 Dollars ($1,000,000.00)
     provided however, the minimum Advance must be at least $50,000.00. The
     Borrower shall have the right to borrow, repay and borrow again under the
     credit facility. The outstanding principal balance of the Liquidity Note on
     July 15, 2000 shall convert to a term facility (the "Term Period") and
     shall be payable in 20 equal quarterly installments of principal plus all
     accrued and unpaid interest, with all unpaid principal plus all accrued and
     unpaid interest being due and payable on July 15, 2005.  Principal and
     interest of the Liquidity Note shall be due and payable as provided in the
     Liquidity Note.

     1.2  Amendment to Negative Covenants.  The Borrower and Lender agree to
          --------------------------------
amend, and do hereby amend, Sections 5.04, 5.06, 5.10 of the Negative Covenants
in the Loan Agreement such that after the amendment such Sections shall read in
their entirety as follows:

          "5.04 Tangible Net Worth.  The Borrower shall not permit its
                ------------------
     consolidated Tangible Net Worth to at any time be less than Thirty Five
     Million and No/100 Dollars ($35,000,000.00) plus the Tangible Net Worth of
     any subsidiaries acquired after Closing Date.  The Borrower shall not
     permit the Bank's Tangible Net Worth at any Time to be less than Fifteen
     Million and No/100 Dollars ($15,000,000.00)."

          "5.06 Dividends.  Borrower shall not declare or pay any dividends,
                ---------
     make any payment on account of any class of the capital stock of Borrower
     now or hereafter outstanding, or make any distribution of cash or property
     to holders of any shares of such stock which exceed $3,500,000.00 in the
     aggregate during any fiscal year, provided however, without Lender's prior
     written consent, Borrower will not declare any dividend so long as Borrower
     is in default in payment of the Obligations.  Payment by Borrower for
     shares purchased under a stock repurchase plan by which Borrower purchases
     its own shares on the open market will not be considered a distribution to
     shareholders."

          "5.10 Limitation on Debt.  Borrower shall not, nor allow any
                ------------------
     Subsidiary to, create, incur, assume, become liable in any manner in
     respect of, or suffer to exist, any debt for borrowed money except:

          (a)  debt, excluding debt created under this Agreement, not in excess
               of $1,000,000.00 at any one time outstanding;
          (b)  debt created under this Agreement;
<PAGE>

          (c)  debt secured by a purchase money security interest; and
          (d)  $10,000,000.00 of federal funds purchased excluding intercompany
               transactions (meaning transaction between or among Borrower and
               its Subsidiaries, or any of them).

     1.3  Amendment to Loan Agreement.  The Borrower and Lender agree to add,
          ----------------------------
and do hereby add, Section 5.13 in the Loan Agreement such that after the
amendment such Section shall read in its entirety as follows:

          "5.13 Loan Loss Reserve.  Borrower shall cause the Bank to maintain at
                -----------------
     all times a reserve for loan losses in an amount not less than 1.0% of
     total loans (on a consolidated basis), to be calculated at the end of each
     fiscal quarter."

                                  ARTICLE II
                                  ----------

                          Conditions of Effectiveness
                          ---------------------------

     2.1  Effective Date.  This Fourth Amendment shall become effective as of
          ---------------
July 15, 1999, when, and only when, Lender shall have received counterparts of
this Fourth Amendment executed and delivered by Borrower and when each of the
following conditions shall have been met, all in form, substance, and date
satisfactory to Lender.

          (a)  Renewal Notes.  Borrower shall have executed and delivered to
               -------------
               Lender a new Acquisition Note and Liquidity Note, payable to the
               order of Lender as set forth therein, duly executed on behalf of
               the Borrower, dated effective July 15, 1999 in the principal
               amounts of $8,000,000.00 and $1,000,000.00, respectively.

          (b)  Additional Loan Documents.  Borrower shall have executed and
               --------------------------
               delivered to Lender such other documents as shall have been
               requested by Lender to renew, and extend, the Loan Documents to
               secure payment of the Obligations of Borrower, all in form
               satisfactory to Lender and its counsel.


                                  ARTICLE III
                                  -----------

                         Representations and Warranties
                         ------------------------------

     3.1  Representations and Warranties.  In order to induce Lender to enter
          -------------------------------
into this Fourth Amendment, Borrower represents and warrants the following:

          (a)  Borrower has the corporate power to execute and deliver this
               Fourth Amendment, the Notes, and other Loan Documents and to
               perform all of its obligations in connection herewith and
               therewith.

          (b)  The execution and delivery by Borrower of this Fourth Amendment,
               the Notes, and other Loan Documents and the performance of its
               obligations in connection herewith and therewith: (i) have been
               duly authorized or will be duly ratified and affirmed by all
               requisite corporate action; (ii) will not violate any provision
               of law, any order of
<PAGE>

               any court or agency of government or the Articles of
               Incorporation or Bylaws of such entity; (iii) will not be in
               conflict with, result in a breach of or constitute (alone or with
               due notice or lapse of time or both) a default under any
               indenture, agreement or other instrument; and (iv) will not
               require any registration with, consent or approval of or other
               action by any federal, state provincial or other governmental
               authority or regulatory body.

          (c)  There is no action, suit or proceeding at law or in equity or by
               or before any governmental instrumentality or other agency or
               regulatory authority now pending or, to the knowledge of
               Borrower, threatened against or affecting Borrower or any
               properties or rights of Borrower or involving this Fourth
               Amendment or the transactions contemplated hereby which, if
               adversely determined, would materially impair the right of
               Borrower, to carry on business substantially as now conducted or
               materially and adversely affect the financial condition of
               Borrower or materially and adversely affect the ability of
               Borrower to consummate the transactions contemplated by this
               Fourth Amendment.

          (d)  The representations and warranties of Borrower contained in the
               Loan Agreement, this Fourth Amendment, the Notes, and any other
               Loan Document securing Borrower's Obligations and indebtedness to
               Lender are correct and accurate on and as of the date hereof as
               though made on and as of the date hereof, except to the extent
               that the facts upon which such representations are based have
               been changed by the transactions herein contemplated.


                                   ARTICLE IV
                                   ----------

                     Ratification of Obligations and Liens
                     -------------------------------------

     4.1  Ratification of Obligation.  The Borrower does here acknowledge,
          ---------------------------
ratify and confirm that it is obligated and indebted to Lender as evidenced by
the Loan Agreement (as amended by the First Amendment, the Second Amendment, the
Third Amendment and this Fourth Amendment), the Notes and all other Loan
Documents.

     4.2  Valid Liens.  Borrower hereby acknowledges and agrees that the liens
          ------------
and security interests of the Loan Documents are valid and subsisting liens and
security interests and are superior to all liens and security interests other
than those exceptions approved by Lender in writing.  Nothing herein contained
shall affect or impair the validity or priority of the liens and security
interests under the Loan Documents.

     4.3  Ratification of Agreements.  The Loan Agreement (as previously amended
          ---------------------------
by the First Amendment, Second Amendment  and the Third Amendment), this Fourth
Amendment, the Notes, and each other Loan Document, as hereby amended, is
acknowledged, ratified and confirmed in all respects as being valid, existing,
and of full force and effect.  Any reference to the Loan Agreement in any Loan
Document shall be deemed to be a reference to the Loan Agreement (as previously
amended) and as amended by this Fourth Amendment.  The execution, delivery and
effectiveness of this Fourth Amendment shall not, except as expressly provided
herein, operate as a waiver of any right, power or remedy of Lender under the
Loan Agreement, nor constitute a waiver of any provision of the Loan Agreement.
The Borrower acknowledges, ratifies and confirms that the collateral securing
the Loan secures all of the indebtedness of the Borrower, including without
limitation, the Notes.
<PAGE>

                                   ARTICLE V
                                   ---------

                                 Miscellaneous
                                 -------------

     5.1  Survival of Agreements.  All representations, warranties, covenants
          -----------------------
and agreements of Borrower, herein or in any other Loan Document shall survive
the execution and delivery of this Fourth Amendment, and the other Loan
Documents and the performance hereof and thereof, including without limitation
the making or granting of the Loan and the delivery of the Notes and all other
Loan Documents, and shall further survive until all of Borrower's Obligations to
Lender are paid in full.  All statements and agreements contained in any
certificate or instrument delivered by Borrower hereunder or under the Loan
Documents to Lender shall be deemed to constitute the respective representations
and warranties by Borrower and/or respective agreements and covenants of
Borrower under this Fourth Amendment and under the Loan Agreement.

     5.2  Loan Document.  This Fourth Amendment, the Notes, and each other Loan
          --------------
Documents executed in connections herewith are each a Loan Document and all
provisions in the Loan Agreement, as amended, pertaining to Loan Documents apply
hereto and thereto.

     5.3  Governing Law.  This Fourth Amendment shall be governed by and
          --------------
construed in all respects in accordance with the laws of the State of Texas and
any applicable laws of the United States of America, including construction,
validity and performance.

     5.4  Counterparts.  This Fourth Amendment may be separately executed in any
          -------------
number of counterparts and by different parties hereto in separate counterparts,
each of which when so executed shall be deemed to constitute one and the same
Fourth Amendment.

     5.5  Release of Claims.  Borrower by its execution of this Fourth
          ------------------
Amendment, hereby declares that it has no set-offs, counterclaims, defenses or
other causes of action against Lender arising out of the Loan, the renewal,
modification and extension of the Loan, any documents mentioned herein or
otherwise; and, to the extent any such setoffs, counterclaims, defenses or other
causes of action which may exist, whether known or unknown, such items are
hereby expressly waived and released by Borrower.

     5.6  Attorneys' Fees.  Borrower hereby agrees to pay Lender, upon demand,
          ----------------
the reasonable attorneys' fees and expenses of Lender's counsel, filing and
recording fees and other reasonable expenses incurred by Lender in connection
with this Fourth Amendment.  Borrower also agrees to provide to Lender such
other documents and instruments as Lender may reasonably request in connection
with the renewal, extension and modification of the Loans mentioned herein.

     5.7  ENTIRE AGREEMENT.  THIS FOURTH AMENDMENT, TOGETHER WITH ANY LOAN
          -----------------
DOCUMENTS EXECUTED IN CONNECTION HEREWITH, CONTAINS THE ENTIRE AGREEMENT BETWEEN
THE PARTIES HERETO RELATING TO THE SUBJECT MATTER HEREOF AND THEREOF AND ALL
PRIOR AGREEMENTS RELATIVE THERETO WHICH ARE NOT CONTAINED HEREIN OR THEREIN ARE
TERMINATED.  THERE ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN THE PARTIES.  THIS
FOURTH AMENDMENT, AND THE LOAN DOCUMENTS MAY BE AMENDED, REVISED, WAIVED,
DISCHARGED, RELEASED OR TERMINATED ONLY BY A WRITTEN INSTRUMENT OR INSTRUCTIONS,
EXECUTED BY THE PARTY AGAINST WHICH ENFORCEMENT OF THE AMENDMENT, REVISION,
WAIVER, DISCHARGE, RELEASE OR TERMINATION IS ASSERTED.  ANY ALLEGED AMENDMENT,
<PAGE>

REVISION, WAIVER, DISCHARGE, RELEASE OR TERMINATION WHICH IS NOT SO DOCUMENTED
SHALL NOT BE EFFECTIVE AS TO ANY PARTY.

     IN WITNESS WHEREOF, this Fourth Amendment is executed effective as of the
date first written above.

BORROWER:                     SUMMIT BANCSHARES, INC.

                                    By:  /s/ Philip E. Norwood
                                        -----------------------------
                                    Its: Chairman
                                        -----------------------------


                                    By:  /s/ Bob G. Scott
                                        -----------------------------
                                    Its: Executive Vice President
                                        -----------------------------



LENDER:                             THE FROST NATIONAL BANK

                                    By:  /s/  Jerry L. Crutsinger
                                        ----- -----------------------
                                    Its: Vice President
                                        -----------------------------