REPORT OF THE DIRECTOR I917 143 



Summary of Geologic History 



Although the Adirondacks offer an attractive field in which to 

 study geology, their very complexity and age are a great hinderance 

 in reaching accurate conclusions, and many steps in their develop- 

 ment remain obscure so that it is impossible to set forth a concise 

 statement giving the exact geological history of the region, but 

 nevertheless a general summary is attempted. 



1 The oldest rocks of the region are the Grenville series of sedi- 

 ments deposited in marine waters. They were composed of sand- 

 stones, limestones and shales. This deposit occurred under 

 conditions of which we know next to nothing, as it perhaps occurred 

 a thousand million years ago. 



(Age, Grenville, the base of the Precambrian) 



2 Following the deposition of the Grenville series, the land experi- 

 enced an uplift that caused its appearance above the surface of the 

 sea, thus 



3 Enabling erosion to remove huge quantities from its mass, and 



4 Followed by a period when a granite intrusion worked its way 

 into the overlying sediments. 



(Age, probably Laurentian) 1 



5 The granite and the Grenville were then subjected to mountain- 

 making disturbances that apparently were more intense in the 

 northwest part of the Adirondacks than in the southeast. These 

 dynamic forces materially altered (metamorphosed) them, folding 

 them together into a very involved mass. 



6 Erosion continuing lowered the altitude by removing huge 

 quantities of all the foregoing rocks. 



7 Then followed the great intrusions of 



a Anorthosite 



b Syenite, with its granitic differential phase 

 c Gabbro 

 (Age, perhaps Algoman) 



8 An uplift of the region followed, accompanied by great vertical 

 compression that altered and foliated all the above-mentioned 

 formations. These met amorphic agencies in contrast to those 

 mentioned in paragraph 5 were most active in the southeast, becom- 

 ing less and less effective toward the northwest. 



9 Erosion continuing through a long period of time gradually 

 reduced the elevation of the entire region. 



1 Recent investigations of the writer show that a basic rock, of Laurentian (?) 

 age, followed the granite. This " metagabbro " has not, as far as the writer 

 knows, been noted in the Lake Clear region. 



