Exhibit 10.68

# FOURTH AMENDMENT TOAGREEMENT OF SALE AND
PURCHASE

THIS
FOURTH AMENDMENT TO AGREEMENT OF SALE AND PURCHASE (this Fourth
Amendment) is entered into as of the 21st day of October,
2004, by and between MACK-CALI TEXAS PROPERTY L.P., a Texas limited partnership
(Seller), and CENTENNIAL ACQUISITION COMPANY, a Texas corporation, and
WARAMAUG ACQUISITION CORP., a Texas corporation (collectively,
Purchaser).

A.              Seller
and Purchaser entered into that certain Agreement of Sale and Purchase           dated as
of August 10, 2004, as amended by that certain Amendment to Agreement           of Sale
and Purchase between Seller and Purchaser dated as of October 12,           2004, as
further amended by that certain Second Amendment to Agreement of Sale           and
Purchase between Seller and Purchaser dated as of October 18, 2004, and           as
further amended by that certain Third Amendment to Agreement of Sale and
          Purchase between Seller and Purchaser dated as of October 20, 2004 (as amended
          to date, the Purchase Agreement). All
          capitalized terms used but not defined herein shall have the meaning given such
          terms in the Purchase Agreement.

B.              Seller
and Purchaser now desire to amend the Purchase Agreement as set forth           below.

NOW,
THEREFORE, for good and valuable consideration, the receipt and sufficiency of which is
hereby acknowledged, Seller and Purchaser agree as follows:

1.    Evaluation
Period.   The expiration of the Evaluation Period           is hereby
extended to 5:00 p.m. Eastern Time on October 25, 2004.

2.    Reaffirmation.  
Except as modified by this Fourth Amendment,           the Purchase Agreement is hereby
ratified and confirmed and in full force and           effect. In the event of a conflict
between the terms of this Fourth Amendment           and the Purchase Agreement, the
terms of this Fourth Amendment shall control.

3.    Counterparts.  
This Fourth Amendment may be executed in any           number of counterparts, all of
which taken together will constitute one and the           same Fourth Amendment, and the
signature page of any counterpart may be removed           therefrom and attached to any
other counterpart. This Fourth Amendment shall be           legally binding upon receipt
by each party of the facsimile or the original           signature of Seller and of
Purchaser.

[SIGNATURES
FOLLOW ON NEXT SUCCEEDING PAGE]

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IN
WITNESS WHEREOF the parties have executed this Fourth Amendment as of the day and year
first above written.

| |  | PURCHASER:CENTENNIAL ACQUISITION COMPANYBy:  /s/ Steven H. Levin
Name:  Steven H. LevinTitle:  President
| |  | ----------------------------------------------------------------------------------------------------------------------

| |  | WARAMAUG ACQUISITION CORP.By:  /s/  Cindy Nelson
Name:  Cindy NelsonTitle:  Vice President
| |  | --------------------------------------------------------------------------------------------------------

| |  | SELLER:MACK-CALI TEXAS PROPERTY L.P.By:  Mack-Cali Sub XVII, Inc., its general partner
      By:  /s/  Roger W. Thomas            Name:  Roger W. Thomas
      Title:  Executive Vice President                   and General Counsel
| |  | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

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