This section is from the book "The National Capitol. Its Architecture Art And History", by George C. Hazelton, Jr. Also available from Amazon: The National Capitol Its Architecture Art and History.
The south wing of the Capitol remains internally in the state in which it was left at the close of the year 1815, excepting in as far as the suggestions of the committee of the House of Representatives . . . have been so far executed as to prepare the south windows of their hall for an access to a platform along the south front. Externally all the injury which was done to the windows and doors by the fire, has been repaired. . . . The Hall of Representatives was so ruined that, although the columns and the vaults they supported still stood, it was inevitably necessary to take them down, so as to clear the whole area of the principal story of the former work".
It seems that, when Congress resolved to repair and rebuild the Capitol, no building materials were to be found in the District. At that time the quarries, which were situated forty miles below the city on the Potomac, had been neglected for some years, and time and much labor would have been required to re-open them.
"For the columns," continues Latrobe in his report, "and for various other parts of the House of Representatives, no free-stone that could be at all admitted has been discovered. Other resources were therefore sought after. A stone hitherto considered only as an encumbrance to agriculture, which exists in inexhaustible quantity at the foot of the most south easterly range of our Atlantic mountains, . . . certainly from the Roanoke to the Schuylkill, and which the present surveyor of the Capitol, and probably others, had many years ago discovered to be very hard but beautiful marbleŚwas examined, and, . has been proved to answer every expectation that was formed, not only of its beauty, but of its capacity to furnish columns of any length, and to be applicable to any purpose to which colored marble can be applied. The present commissioner of the public buildings has, therefore, entered into a contract for all the columns, and progress has been made in quarrying them; They may be produced each of a single block. . . . The quarries are situated in Loudon County, Virginia, and Montgomery County, Maryland.