192 NEW YORK STATE MUSEUM 



the Grenville series are about equally extensive. An important 

 feature is the relation of the igneous and sedimentary masses 

 because the igneous rocks, though intrusive as usual, here break 

 through the sediments in numerous small to large separated masses 

 which gives rise to a distinct patchwork effect much more per- 

 fectly shown on the geologic map here than on any other Adirondack 

 map so far published. A good idea of this patchwork effect is 

 given by the accompanying detailed geologic maps on which it will 

 be seen that the igneous masses are often nearly or completely sur- 

 rounded by the Grenville. The protrusion of these very resistant 

 igneous rocks through the weak Grenville is a primary considera- 

 tion because, as a result of long erosion, the hard igneous masses 

 have stood out as mountains above the worn down Grenville, and 

 thus the way has been prepared for the development of exfoliation 

 domes. The North Creek quadrangle shows an almost perfect 

 adjustment of topography to rock character. 



2 Faulting. That the eastern Adirondacks are considerably 

 faulted has been recognized for some years, but, thus far, little 

 attention has been paid to the detailed study and mapping of these 

 faults well within the Precambric area. As a rule the faults are 

 difficult to locate with any great degree of accuracy and certainty, 

 but in Warren county there is a good opportunity for their study. 

 Frequently the line of contact between the syenite or granite and 

 the Grenville is very regular and sharp, the Grenville seeming to 

 dip under the igneous rock with the latter rising very abruptly 

 and to a great height above the Grenville. Among the best exam- 

 ples of this phenomenon are the southern sides of Huckleberry, 

 Crane, and Little mountains and the western sides of Birch and 

 Potter mountains. There are only two possible explanations of this 

 phenomenon, namely, either that the igneous rocks were intruded 

 in the position which they now occupy or that faulting has occurred. 

 If this is to be explained simply on the basis of intrusion then we 

 are forced to assume a remarkably irregular surface of the newly 

 cooled magma and also that the molten masses, in all of these cases, 

 broke through the Grenville along very straight or regular lines 

 often for miles. Both of these assumptions are entirely out of har- 

 mony with well-known observations in other regions. Among the 

 positive evidences for faulting are the frequent presence of sheared 

 or brecciated zones along the lines ; the fact that these blocks 

 always show a distinct tilting away from the crests of the scarps; 

 and the well-known faults along Lake Champlain and in the 



