BASAL GNEISSES OF THE HIGHLANDS 373 



types in the qiiartzite. both Cambric and Lower Sikiric in the 

 limestones and Lower SiUiric in the slates. The quartzite is the 

 true " Poughqiiag." 



In Peekskill Creek valley and along the south margin of the 

 Highlands, southwest of Tompkins Cove, the same relations pre- 

 vail in every respect except that there is later faulting and close fold- 

 ing in this belt. The group, however, is not crumpled and 

 contorted into the complexities prevalent in the lower forma- 

 tions. Only where it comes into contact with the Cortlandt 

 intrusion does it become at all complex in structure or petro- 

 graphic character. 



For these reasons, together with the support given by the 

 fault structure next to be described, but stripped of most of 

 the details of local observational descriptions, it is the writer's 

 opinion that there are two distinct groups of formations above 

 the basal gneisses in the Highlands region. The older and more 

 complex, wholly cr3^stalline. at the base a limestone (Inwood) 

 followed by a schist (Manhattan) both of Precambric age, 

 occupies together with the gneisses almost the whole of the 

 Highlands and the southward extension to New York city. The 

 younger, a Cambro-Siluric series with a thick quartzite always 

 at the base (Poughquag) followed b}^ a limestone (Wappinger) 

 and completed by a slate (Hudson River) forms a continuous 

 border along the north of the Highlands and occurs in only 

 an occasional fault valley along the southern border, notably 

 Peekskill Creek valley, its extension southward, and a few adja- 

 cent localities. 



Structural features of the region 



Folding is evident everywhere. The gneisses almost always 

 stand at very steep angles, vertical or nearly so, and sometimes 

 are overturned, all evidently the eroded edges of large folds. 

 Where proximity to or contact with overlying formations aids 

 in forming a conception of the correct superposition, it appears 

 that the overturning is chiefly toward the northwest. 



The general strike of all the major folds is northeast and 

 southwest. But in the vicinity of the Cortlandt igneous mass 

 the strike is nearly east and west. Besides there are occasional 

 cross-foldings on large enough scale to radically change the 

 strike for considerable distances. Minor crumplings indicate 

 a common tendency of this sort. A general cross-folding effect 



