GEOLOGY OF THE NORTH CREEK QUADRANGLE 47 



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

 these cases, broke through the Grenville along very straight or regu- 

 lar lines often for miles. Both of these assumptions are out of 

 harmony with well-known observations in other regions. 



The very common dip of the Grenville downward against the 

 faults is due to the fact that most of the prominent fault scarps 

 face the west wiiile the prevailing dip of the Grenville is toward 

 the east. 



Among the more positive criteria for the recognition of the faults 

 are the following : ( i ) actual steep to vertical scarps, often in hard, 

 perfectly homogeneous rock, and frequently in such positions as to 

 preclude the possibility of their having been formed by ice or stream 

 erosion; (2) the distinct tilting of the earth blocks gradually down- 

 ward away from the scarps; (3) the frequent presence of actual 

 crushed, sheared, or brecciated fault zones; and (4) the long, 

 straight contact lines between the Grenville and the igneous rocks, 

 with the latter rising abruptly high above the former. 



What is the age of the faulting? That some, at least, occurred 

 during Precambric time has been pretty well established but, so tar 

 as known, such faults are of minor importance and certainly have 

 no appreciable influence upon existing topography. But a single 

 case of such very ancient faulting has come under the writer's 

 notice within the area of the quadrangle and this occurs along the 

 road three-fifths of a mile southwest of Sullivan pond. A fault, 

 plainly visible for 12 feet, there passes across a glaciated ledge of 

 quartz syenite. On the east side, for a width of 7 or 8 feet, the 

 whole mass is a fault breccia. This breccia is fine at the fault and 

 coarser, with fragments up to i V2 feet across, away from the fault. 

 The fault strikes north 30° east and is in no way related to existing 

 topography. 



There is good reason to think that considerable faulting occurred 

 during the Paleozoic era after the deposition of the Ordovicic sedi- 

 ments because rocks of that age are involved in the faulting along 

 the eastern and southern borders of the Adirondacks. Gushing has 

 suggested that the faulting may have been initiated at the time of 

 the Taconic revolution when the rocks immediately eastward were 

 so greatly disturbed, but he says:' "The great earth disturbance 

 (Appalachian revolution) which prevailed in the Appalachian zone 

 toward the close of the Paleozoic would seem more likely to have 

 brought about the major faulting of the reg^ion.'' We know that 



>• N. Y. State Mus. Bui. 95, p. 405. 



