388 Merrill — Metarnoiphic Strata of S. E. New York. 



the earth's history as the sea water was inhabited by creatures 

 possessing skeletons of carbonate of lime. The source of the 

 magnesia so abundant in the limestone is yet undetermined. 



The lowest stratum yet recognized among the Westchester 

 County rocks is a reddish gneiss which forms the central mass 

 of some of the hills in Yonkers where it is well exposed, and 

 consists of small grains of detrital quartz with fragments of 

 reddish orthoclase and a few crystals of biotite which have 

 developed during the process of metamorphism. From its 

 macroscopical characters the rock would be called a gneiss. 

 From its microscopic structure it would be called a metamor- 

 phosed sandstone or arkose. As the name quartzite gneiss has 

 been applied in Germany to rocks of analogous structure, it is 

 proposed to designate the rock in question as an arkose gneiss. 

 It may be objected that all gneisses Avere quartzites or arkoses 

 previous to their metamorphism, but there is an evident neces- 

 sity for the use of some descriptive term which will convey 

 the idea of comparatively unaltered detrital structure and 

 differentiate such a rock as this from the pre-Cambrian gran- 

 itoid gneisses of Putnam County. 



The thickness of the red gneiss referred to has not been 

 determined, as no section has been found which shows its rela- 

 tions to the formation beneath it, but it is believed to rest 

 upon th# stratified granulites which form the second or upper 

 member of the pre-Cambrian formation. Since the best ex- 

 posures of the red gneiss have been found within the limits of 

 Yonkers and its relations to the overlying stratum are well 

 shown within the limits of that city, it is proposed to call it 

 the Yonkers gneiss. Outcrops of this rock are frequent along 

 the shores of the Hudson and good exposures may be found at 

 Hastings, on the property of Dr. Draper ; on the river near 

 the southern border of Tarrytown ; between Scarborough and 

 Sing Sing stations near the railroad ; along the southeast shore 

 of the mouth of Croton Bay on the property of Orlando B. 

 Potter, Esq. ; and a little south of Oscawana Station, on Osca- 

 wana Island ; it also probably occurs in the ridge which sepa- 

 rates Annsville Cove from the valley of Peekskill village. 

 The best exposures are in Yonkers, on and near Jerome 

 avenue, a little north of the New York City line. 



With regard to the variations in the composition and struc- 

 ture of the Yonkers gneiss, the most important are an increase 

 in the proportion and size of the feldspar fragments in ap- 

 proaching the Lauren tian Highlands. 



Overlying the reddish Yonkers gneiss and beneath the crys- 

 talline limestone is a stratum of thinly bedded gray quartzite 

 gneiss. This contains but little feldspar and its component 

 beds vary in composition from almost pure quartz to a mixture 



