244 



liara Robinson, of Baltimore, and at the same time had visited 

 the quarries in the same town where the marbles now employed 

 in the extension of the General Post-Office and Patent-Office at 

 Washington, wei'e obtained. The marbles at Mr. Robinson's 

 quarries are identical in character with those from the United 

 States quarries above mentioned. 



The Dolomite, which is the finer-grained marble, similar to 

 that used in the Post-Office extension, has a density of 2.851, 

 hence a cubic foot of it will weigh 178.187 pounds avoirdupois. 



On chemical analysis this stone is found to consist of 



Carbonate of lime -------- 59.4 



Carbonate of Magnesia ------ 38.5 



Carbonate of Manganese and Iron - - - 1.4 



Insoluble siliceous matter ------ 0.7 



100.0 



The strength of a stone of the neighboring quarry, tested at 

 Washington, was found to be equal to a resistance of 18,061 

 pounds per square inch. This marble is suitable both for monu- 

 mental purposes and for architecture. The broken fragments of 

 it make, when burned, a hot magnesian lime, which, when mixed 

 with sand, forms a mortar with some hydraulic properties, so that 

 it is very permanent, and resists the action of water after it is 

 once hardened or set. 



The other marble is a coarse-grained white limestone, called 

 by the quarrymen Alum Stone, on account of the large size of its 

 crystals, and their great purity. A stone identical with this is 

 employed in the extension of the Patent-Office buildings. This 

 marble has a density of 2.697, and a cubic foot of it weighs 

 168.562 pounds. Its strength is equal to a resistance of 8.057 

 pounds per square inch, according to the experiments made at 

 Washington on a stone identical with this which was taken from 

 a neighboring quarry. 



This stone is largely burned for making lime, and furnishes the 

 best quality of pure white lime for mortar and for plastering ceil- 

 ings. The marble is also employed in building, though it is far 

 inferior in strength to the dolomitic variety before described. 

 On chemical analysis this limestone was found to consist of 



