376 NEW YORK STATE MUSEUM 



and is a considerable support for the theory of two separate 

 sedimentary groups. These larger faults are believed to be 

 chiefly responsible for the sudden disappearance of the Man- 

 hattan schist. 



If there be, as seems reasonable., a north and south fault 

 in the Hudson river also from Peekskill southward, it will be 

 noted that such a line would intersect the one just described 

 in the Peekskill area. Most suggestive is the occurrence of the 

 Cortlandt igneous intrusion just at the acute angle of the 

 depressed block thus outlined. On every margin of the Cort- 

 landt area are evidences of faulting, fault breccias, shear zones, 

 -clear-cut faults, great inclusions, and abrupt transitions. That 

 this igneous outbreak is genetically connected with the develop- 

 ment of excessive weakness at this point by the block and thrust 

 faulting of the district appears to give an explanation of its 

 limited areal distribution and its occurrence at a time not 

 marked by igneous activity in other unrelated areas. 



Age of the basal gneisses 



The age and exact correlation of this lowest formation is 

 unknown. It has all the physical character of the " Grenville 

 series " of the Adirondacks and Canada as described by geol- 

 ogists in those districts and as seen by the writer at a few 

 points in the Adirondacks. In view of the agreement in petro- 

 graphic character and general stratigraphic features it is believed 

 to be a part of the " Grenville series." It is surely not Archean, 

 in the present meaning of that term as now applied in the 

 Great Lakes region of the United States and Canada. 



The relative age of this formation is more clear. It is the 

 oldest of the immediate region under discussion. It is wholly 

 Precambric. It is, in the writer's opinion, separated from the 

 lowest representatives of the Cambric shown above by one 

 overlap unconformity probably of no very great time break, a 

 group of sediments (Inwood limestone and Manhattan schist) 

 of great thickness, and an unconformity marked by mountain 

 folding and erosion. Therefore it is immensely older than the 

 Cambric and its stratigraphic position under this conception of 

 the relations between the overlying formations may be tabulated 

 as follows : 



