278 NEW YOEK STATE MUSEUM 



Later Precambric disturbances. During the long Precambric 

 erosion period the present surface ro-cks were gradually approach- 

 ing the surface as the overlying rocks were, bit by bit, removed. 

 They were therefore under progressively less and less load of 

 overlying rocks, and, if subjected to compression during this stage, 

 the effects produced would be very different from those brought 

 about by compression under great load. That the rocks were so 

 affected when much nearer the surface is clear, the main result 

 being the production of the highly inclined or vertical, rather even 

 cracks or fissures known as joints. There have been later times 

 of joint formation in the region also, and the different sets are 

 difficult of discrimination. But it is. clear that there was some 

 development of joints and faults, indicative of stress, at this time. 



late Precambric igneous activity. During most of the long 

 period of denudation which followed the time of the great igneous 

 intrusions there was an absence of igneous activity in the region, 

 at least in so far as near surface effects were concerned. But 

 toward the close of the period, when the present surface rocks were 

 no longer deeply buried, but were comparatively near the surface, 

 molten rock again came up from benearth, likely from the same 

 source whence the material of the great intrusions sprang. 

 Whether any of this molten rock reached the surface then existing 

 can not be determined, since no vestige of that old surface 

 now remains, but in all probability there was volcanic action at 

 the surface. The lavas utilized a sysitem of east and west fissures 

 or joints as their channels of ascent and eventually cooled and 

 solidified in them. Such lava-filled fissures are known as dikes, 

 and these dikes are very numerous in portions of the region, 

 specially at the northeast. Their upper parts, along with their 

 surface outpourings, were worn away long ago. Could they be 

 followed in depth, they would lead eventually to the reservoirs 

 which supplied the material with which they are filled. 



There are two sets of these dikes, showing that there were at 

 least two separate periods of igneous action at the time. The 

 more common dikes are of heavy, dense, black rocks of the sort 

 known as diabase. The others are less dense and heavy, usually 



