612 REPORTS OF THE BUILDING COMMITTEE. 



roof, that a rough floor he laid upon the heams above the upper ceiling, next to the 

 roof, throughout the building, and covered with a suitable thickness of lime-water 

 cement ; over which, when dried, a covering of common salt, or a cement of salt 

 and ashes, be evenly spread, carefully filling all the cracks or crevices of the cement. 



This is the French method of fire-proofing the upper part of buildings, and has 

 been to some extent adopted in some sections of this country with entire success. 

 This would be attended with comparatively little expense, and would, it is believed, 

 render the body of the building entirely secure against destruction by fire in case the 

 roof should be burnt. 



All of which is respectfully submitted, 



¥M. J. HOUGH. 



"Washington, March 15, 1847. 



On motion of Mr. Seaton, it was — 



Resolved, That Mr. Hough be requested to examine into the expediency of dispens- 

 ing with any portion of ornamental stucco in the building. 



The chairman submitted from Dr. Owen the following report on the sand-stones of 

 the Potomac : 



After completing the report of the white marbles and granite of Baltimore county, 

 Maryland, I proceeded, according to instructions, on the afternoon of the 11th of 

 March, to inspect the quarries of sand-stone in the neighborhood of Seneca creek, 

 Montgomery county, Maryland. 



About 21 J miles from Washington city, on the line of the Chesapeake and Ohio 

 canal, the talcose and chloritic schists of the Great Falls of the Potomac are suc- 

 ceeded by freestone and marly beds of the new red sandstone formation. These 

 deposits are of various colors, from a' light, greenish-gray, or dove-color, to a deep 

 red or brown. 



The first quarry visited is situated on Bull Kun, 23 miles from Washington. The 

 excavations have hitherto been carried on from 250 to 300 yards on the north side of 

 the canal ; but the ledges of rock extend down to its margin, and can be quarried 

 with nearly as much facility a few paces from the canal as where operations are now 

 carried on. 



The dip of the stratification slopes at an angle of 15° or 20° upwards from Bull 

 Pun. The beds, suitable, both as regards color, durability, and ease with which they 

 can be cut, are fortunately near the surface in the ravine of Bull Run. By com- 

 mencing quarrying operations near the bed of this stream, and working up the west- 

 ern slope, blocks of large dimensions can be obtained with comparatively little labor. 

 The beds which have been chiefly worked here are layers of a deep red color, (see 

 specimen No. 18,) and layers of a purplish-gray, (No. 19,) which, by exposure, 

 acquire a lighter and more pinky hue. The latter is the rock most suitable for build- 

 ing purposes, its color being agreeable, and, in the opinion of men of good judgment 

 and taste, appropriate for the Norman style of architecture. This rock possesses one 

 property in particular which recommends it to the attention of builders. When first 

 removed from the parent bed, it is comparatively soft, working freely before the 

 chisel and hammer, and can even be cut with a knife; by exposure, it gradually 

 indurates, and ultimately acquires a toughness and consistency that not onlj T enables 

 it to resist atmospheric vicissitudes, but even the most severe mechanical wear and 

 tear. Abundant evidence of this is afforded in the buildings of the neighborhood, 

 in several of the locks and aqueducts, and also in ledges and blocks exposed in the 

 bed of Bull Run. The deep red varieties have been chiefly used in these structures. 

 By close inspection of slabs exposed now twenty years to atmospheric agencies and 

 severe mechanical friction, the mark of the dressing chisel is still sharply imprinted 

 in the surface. On the perpendicular wall of the aqueduct, where the water has been 

 oozing through the joints and trickling down its face, forming an incrustation of 

 carbonate of lime, one may observe, where this calcareous crust has scaled ofl*, the 

 grooves and ridges of the surface still nearly as distinct as when the block first came 

 from the hand of the stone-cutter. 



The angles and edges of the keystones of the arch, placed under these most 

 unfavorable circumstances, are sharp and entire. Only one or two blocks of this 

 work of 20 years' standing show signs of decay ; but these seem to be such as either 

 have not been well selected, or have been placed on the edge in the wall. 



Even the tow-path of this aqueduct, over which the horses and mules have been 

 travelling for 20 years, is still unimpaired. Even the corners around which the 

 heavy lock-gates swing, show no signs of chipping. Blocks were pointed out to me 

 in the bed of Bull run, which had been rejected by the engineers as being of too soft 



