54 NEW YORK STATE MUSEUM 



a series of water-deposited rocks, must have been laid down on 

 some floor of preexisting rocks. So far as we can find, no trace of 

 this floor remains and we are in entire ignorance as to its nature. 

 That there must have been a long period of time anterior to this 

 in the earth's history, we are certain, but we entirely lack direct 

 evidence regarding it. 



The Grenville is an enormously thick sedimentary series, com- 

 prising masses of limestone, of shale and of sandstone. It is so 

 thick and so varied that it is quite probable that it comprises more 

 than one formation. To deposit such a thick accumulation of sedi- 

 ments must have required a very great lapse of time. The Gren- 

 ville is as old a rock series as is anywhere known. 



Following the Grenville the region was invaded from beneath 

 by enormous masses of molten rock which were working their 

 w^ay upward toward the surface. They badly broke up the Gren- 

 ville deposits, thrusting their way through them, pushing, them aside, 

 and likely wholly engulfing great masses of them. This intrusion, 

 and those which followed, are no doubt chiefly responsible for the 

 disappearance of the old floor of deposit of the Grenville. The 

 intrusive consisted of granite. It greatly added to the pressures 

 and the temperature of the invaded rocks, and was an important 

 factor in changing them into the crystalline rocks of which they, 

 today, consist. The Grenville rocks today invariably rest upon 

 these intrusives, which are nevertheless younger, in spite of the 

 fact that they underlie them. 



The Grenville rocks and the Laurentian granites (as these early 

 granitic intrusives are called) are early Precambrian. Elsewhere, 

 in Ontario and the States which border on Lake Superior, other and 

 younger Precambrian formations are found : at least three great 

 series of formations younger than the Grenville, aggregating many 

 thousand feet in thickness, and separated from one another by pro- 

 found unconformities, representing times of uplift and of erosion, 

 times apparently as long as those in which the deposits were being 

 laid down. Erosion is a slow process ; so is deposition ; yet thou- 

 sands of feet in thickness of deposits were formed, and a vast 

 amount of material was eroded during that part of the Precambrian 

 which followed the Grenville. 



These younger Precambrian rocks, that is, younger than the 

 Grenville and the Laurentian granite, are, in their turn, accom- 

 panied by, or cut by, intrusives, which are younger than they are. 

 Younger intrusives are also present in the Adirondacks. though 

 the sediments are lacking. A great series of intrusives, anorthosite, 



