GEOLOGY OF THE NEW YORK CITY AQUEDUCT 47 



Ordovicic). The formation is prevailingly of a compact, fine 

 texture, dark gray, either massive or strongly bedded limestone. 

 Where the stratification is very plain there are light and dark layers 

 and an abundant silicious intermixture. In many outcrops the rock 

 is so massive that even the dip and strike are obscure. Some places 

 the rock is fine crystalline, almost a micromarble. On weathered 

 surfaces it almost always exhibits a crisscross etching which marks 

 the traces of rehealed cracks. From these it is seen that many of 

 the apparently massive compact beds have at one time been exten- 

 sively crushed. In many places there is scarcely a square inch 

 wholly free from these evidences. The formation is best exposed 

 in the wide belt that extends southwestward from the vicinity of 

 Poughkeepsie and crosses the Hudson at New Hamburg into the 

 Xewburgh district. It undoubtedly underlies the slates in the rest 

 of the adjacent area. There are few fossils and they are rarely 

 found. 



(24) Poughqiiag qiiartzitc. Below the Wappinger limestone and 

 upon the upturned and eroded edges of the Highlands gneisses lies 

 a quartzite of variaWe thickness but which reaches at least 600 feet. 

 It is a strongly silicified quartz sandstone — a quartzite by indura- 

 tion. It is strongly bedded but seldom shaly. Traces of schistosity 

 may appear in certain zones and this is somewhat strongly developed 

 outside of the area at the type locality (Poughquag, N. Y.). 



Only fragments of trilobite spines have been found in this forma- 

 tion within the district. 



g Later crystallines south of the Highlands. South of the 

 Highlands proper except at one locality (Peekskill creek valley and 

 its southwestward continuation through Tompkins Cove and Stony 

 Point) the rocks are all much more thoroughly crystalline. There 

 are two formations, and in places traces of a third, above the Gren- 

 ville gneisses (Fordham gneisses and associates). These are known 

 locally as Manhattan schist, In-zvood limestone, and Lozverre quartz- 

 ite. In Westchester and Xew York counties the quartzite 

 is rarely found, and in a considerable proportion of those places 

 where it does occur its relations are more consistent with the 

 gneisses ^below than with the limestone-schist series above. This 

 is true indeed of the type locality (Lowerre). There are, however, 

 at least two points where the occurrence favors the reverse inter- 

 pretation, so far as any is shown, and therefore a quartzite may be 

 regarded as finishing the series, and making uncertain but probably 

 unconformable contact with the underlying gneisses. 



