4 PROCEEDINGS OF THE NATIONAL MUSEUM vol.66 



in this State (Maryland). The roclf, which belongs geologically to the Tri- 

 assic formations, is composed of rounded and angular fragments of all sizes, 

 up to several inches in diameter, of quartz and magnesian limestone imbedded 

 in a fine gray calcareous groundmass. This composition renders the proper 

 dressing of the stone a matter of some difficulty, since the hard quartz pebbles 

 break away from the softer parts in which they lie, leaving numerous cavities 

 to be filled with colored wax or shellac. It should therefore never be worked 

 with hammer and chisel, but only with saw and grinding material, and no 

 attempt made at other than plain surfaces. The stone was used for the pillars 

 of the old Hall of Representatives in the Capitol at Washington, and a polished 

 slab 34 inches long by 20 inches wide may be seen in the National Museum at 

 Washington. The pebbles forming the stone are of so varied shades that to 

 state its exact color is a matter of difficulty. Red, white, and slate-gray are 

 perhaps the prevailing tints. On account of its locality the stone has been 

 popularly called " Potomac " marble, or sometimes calico marble, in reference 

 to its structure and spotted appearance. The formation from whence it is 

 derived is said to commence near the mouth of the Monocacy River, and to 

 extend along the Potomac to Point of Rocks and along the valley on the eastern 

 side of the Catoctin Mountain to within 2 miles of Frederick. The writer is 

 informed, moreover, that the same formation occurs in Virginia, near Lees- 

 burg, and that here the quartzose pebbles are almost entirely lacking, thereby 

 rendering the stone less difficult to work. 



At the Leesburg quarry the rock consists of pebbles of white, shity 

 blue or buff fine to coarse-grained marble in a light colored calcareous 

 matrix, so that the general tone of the rock is light colored with no 

 red tints. No quartzose or siliceous pebbles are to be seen and, where 

 the silica content increases it is apparently due to secondary intro- 

 duction of diopside and other silicates. 



THE BASALT 



The basalt is exposed on the east side near the bottom of the quarry 

 where it occurs apparently as two flat easterly dipping dikes about a 

 meter in thickness, separated by several meters of diopside rock. The 

 dike rock apparently has been shattered in part by later faulting 

 which took place at various times and some of the basalt was prob- 

 ably broken up and dragged as fragments into the sheared material, 

 subsequent to its consolidation yet previous to the alteration of the 

 limestone to diopside rock. 



In the hand specimen the rock is medium dark purplish gray in 

 color and dense in structure, no individual minerals being distin- 

 guishable under a lens. It is practically lusterless in the crystalline 

 portion but varies to waxy-lustered in the glassy chilled border 

 phases. The dikes are so jointed that it is difficult to secure a piece 

 large enough to trim into a hand specimen. 



Under the microscope the average rock from the dikes is a very 

 fine grained holocrystalline aggregate of feldspar and pyroxene, 

 both of which tend to assume euhedral form, the pyroxene in short 



