62 NEW YORK STATE MUSEUM 



The distribution of the KjO and HoO is entirely arbitrary. No 

 account is taken of the garnet in this recast. 



b Inzvood limestone. The Inwood Hmestone outcrops in a belt 

 about five miles long, striking approximately East- West, in the south- 

 east part of the quadrangle in Yorktown. The best exposures are 

 at Amawalk and at the cross roads east of Mohansic lake. The lime- 

 stone is comparatively nonresistant to weathering so that outcrops 

 are few and inconspicuous. Open valleys tend to develop on this 

 formation. 



It is a white limestone stained yellowish near the surface. The 

 fresh fracture gHstens with shiny mica scales. It is of medium 

 coarseness with rounded or equant grains and crumbles readily in 

 the hand to a rather coarse sand. 



In thin section it is a coarse-grained limestone carrying muscovite 

 or phlogopite and a little tremolite. The calcite grains are rather 

 simple in form, not well interlocked. They show the effect of 

 dynamic stress in the twinning bands and the well-defined cleavage 

 cracks. The mica is a later development. There is no good evidence 

 here of contact metamorphism. 



The Cambro-Ordovician sediments. Two patches of Cambro- 

 Ordivician sediments are represented on this map ; one on the north 

 margin of the Highlands at the fool of Breakneck ridge where a part 

 of the lowland of the great valley comes within the boundaries of 

 this sheet; the other place is along Peekskill Hollow creek. The 

 types of sediments are represented by quartzites, limestones, shales, 

 slates, phyllites and graywackes, belonging to three distinct forma- 

 tions as follows : 



a Poughquag qiiarfcitc. This rock is developed in beds approxi- 

 mating 600 feet in thickness. It is in general an almost pure quart- 

 zite. It has been completely indurated and to some degree recrystal- 

 lized. Certain beds have an appreciable feldspathic content and 

 others have a carbonate intermixture in considerable amount. It has 

 enough iron also to give it a little color so that chemically it is not 

 quite such pure silica as the appearance of the rock would at first 

 lead one to believe. 



There are no unusual petrographic features or characteristics 

 introducing special questions or uncertainties of interpretation. 



b The Wappinger limestone. This formation is represented by 

 a finely crystalline limestone. The beds lie immediately above the 

 Poughquag in conformalble relation and are developed to a thickness 

 of approximately 1000 feet. In no place in this district, however, 

 is the thickness determinable, except as an interpretation from the 



