164 Geulugy of liicliinond County, X. Y. 



there is, appears to be nmscovite. In places, the hist named 

 mineral is absent, the rock being then a kind of pegmatite or 

 •graphic granite. No stratification is observable, but the surface 

 of the rock outcrop dips about fifteen degrees to the east. Ma- 

 ther calls this granite primary, and to the best of our present 

 knowledge it belongs to the oldest geological formation in North 

 America. 



Steatitie Rocks. — As before mentioned, intignesian rocks, ser- 

 pentines, form the tops, at least, of the main series of hills on 

 Staten Island. It is probable that this rock originally was of 

 very considerable thickness, for a large amount must have been 

 removed by erosion; but yet no granite nor gneiss, which are as- 

 sumed to underlie it, has been seen in j)lace within the serpen- 

 tine area, which is estimated at about 13.5 square miles. The 

 present thickness it is impossible to estimate accurately, but 

 judging from the exposures, I should place it over one hundred 

 feet. 



The most eastern exposed boundary of the serj^entine is clearly 

 and unmistakably marked by a series of very sharp slopes, which 

 are nearly continuous from Tompkinsville to Richmond, and in 

 some places ai'e as straight and regular as they could be con- 

 structed. This regularity of the slope seems to be quite charac- 

 teristic of these hills, and is not the least element of their beauty. 

 How far east of the foot of these hills the serpentine extends is 

 not known, but it is probably not a great distance, as the granite 

 at Tompkinsville occurs within a few hundred feet of it. The 

 southern end of the ridge descends rather gradually, and near 

 Eichmond is lost under the Freshkill marshes. The western 

 boundary of the formation, or more properly the eastern limit 

 of the Triassic sandstone which rests upon it, cannot be accu- 

 rately located, as there are no outcrops, and the line as drawn 

 on the map must be considered as only approximately correct. 



The magnesian rock varies in color from light green to nearly 

 black, and in texture from compact to quite earthy — much of it 

 being fibrous. Its specific gravity is about 2.55, and in chemi- 

 cal composition it is all a hydrated magnesian silicate. The best 

 exposures are at several places around the base of Pavilion Hill 

 at Tompkinsville ; in cuttings for streets in the village of New 



