BASAL GNEISSES OF THE HIGHLANDS 369 



4 A fine grained blue and white banded limestone (iooo feet 

 Wappinger) 



3 A fine grained quartzite (300 to 600 feet, Poughquag) 



2 A coarsely crystalline mica schist, pegmatitic (very thick 

 Manhattan) 



1 A coarsely crystalline limestone, tremolitic, micaceous, peg- 

 matitic (200 to 800 feet, Inwood) 



o A thin schistose quartzite essentially part of the gneiss (1 

 to 100 feet, Lowerre) 



Numbers 1 and 2 forming a group, are strictly conformable 

 to each other, and so are nos. 3, 4 and 5. The two groups 

 so far as the writer is aware, are not at any place in direct 

 contact with each other although such relationship may obtain 

 beneath the water level at the mouth of Peekskill creek. 



This locality including Peekskill Creek valley and Sprout 

 Brook valley are believed to exhibit features of the greatest 

 significance. From the junction of the two streams about a 

 mile northwest of Peekskill, the two valleys diverge toward 

 the northeast. The lower 3 or 4 miles of each valley carries 

 the formations that are considered by the writer typical of the 

 two groups whose relationships are so obscure. Along the 

 ridge between the two valleys passes a great fault which has 

 allowed the block on the southeast of this line, i. e. the Peeks- 

 kill Creek valley and adjacent territory, to drop much lower 

 than the northwestern side on which Sprout brook is located. 

 [See discussion of structural features] Therefore, in these two 

 adjacent valleys, formations of very different type occur so near 

 together that their differences must be considered fundamental. 

 In Peekskill Creek valley the typical 3, 4, 5 group occurs — 500 

 feet of quartzite, probably 1000 feet of limestone, and a great 

 thickness of phyllite. This is, without the slightest doubt, the 

 Poughquag quartzite-Wappinger limestone-Hudson River slate 

 group. It is worthy of special note that even on this southern 

 margin of the Highlands these formations have not lost any 

 of their usual characteristics so well exhibited north of the 

 Highlands. The contacts here are fault planes. The whole 

 group is gradually cut out in passing northeastward, the higher 

 beds disappearing first and the quartzite being traceable for 

 at least 10 miles along this valley. 



In Sprout Brook valley, only a mile across the ridge, lime- 

 stone alone occurs. Because of its proximity to the limestone 



