28 Featherstonhaugtts Geological Report. 



vertical inclination. It is a red granular rock, containing little 

 or no mica, resembling granite in every particular except its 

 stratification. Gneiss is a very abundant rock in the Atlantic 

 portion of the United States. It is extensively quarried in 

 the vicinity of New York and Philadelphia for architectural 

 purposes, whilst on the hills above Georgetown, in the Dis- 

 trict of Columbia, it is in a decomposed state ; the felspar 

 having lost its cohesion, and the mica being very much 

 broken down. The ferruginous matter belonging to this 

 rock has accumulated by molecular attraction, showing itself 

 in intersecting carbonaceous-looking streaks and seams where 

 the roads have been cut through the hills, but being more 

 generally diffused as a red oxyde. These numerous ferru- 

 ginous partings, which cause the gneiss to separate into irre- 

 gular masses with bright black faces, are not found in that 

 part of the gneiss which is undecomposed in the same vicinity 

 along Rock creek. 



The characteristic rock of the mica slate formation con- 

 tains little or no felspar, and has a slaty structure, with a 

 glittering appearance. Its fissility enables it to be conve- 

 niently used for flags on the sideways of streets. It is found 

 in the vicinity of all the gneiss districts. 



Primordial limestone is found in this part of the series, 

 but is not constant to it. In some instances it has been called 

 saccharine limestone, from its white granular resemblance 

 to loaf sugar. It is the material used for statuary marbles, 

 the most valuable beds of which, those that are unmixed with 

 siliceous particles, are very rare. Immense blocks, weighing 

 several tons, have been sometimes transported from Italy to 

 England for the purpose of making a single statue, which, on 

 account of the nests of siliceous matter found in them, have 

 proved almost valueless when worked into. No extensive 

 deposites of it of a good quality have yet been found in the 

 United States. There are many varieties of this saccharine 

 limestone, however, here. It sometimes contains mica, and is 





