76 NEW YORK STATE MUSEUM 



SUMMARY OF GEOLOGICAL HISTORY 



A brief outline of the geological history of the portion of the 

 Adirondacks covered by the North Creek sheet is here given in 

 order to bring together the principal events of that history in the 

 regular order of their occurrence so far as known. 



The oldest known records of the region are written in the Gren- 

 ville rocks which, by their very character and composition, are 

 undoubted metamorphosed sediments. Because of the great thick- 

 ness and widespread distribution of these strata in New York and 

 Canada, we know they were deposited in an extensive ocean and 

 that the length of time required for this sedimentation must have 

 been at least some millions of years. While it is as yet impossible 

 definitely to classify the Grenville with either the Archean or Al- 

 gonkian (Proterozoic), the evidence is decidedly against its late. 

 Precambric age. 



After the 'deposition of the Grenville sediments, the whole Adi- 

 rondack region, including the area of the North Creek sheet, was 

 elevated well above sea level. At this time the Grenville strata 

 were probably folded. Great masses of molten rock, now repre- 

 sented by the syenite, granite, and granite porphyry, were intruded 

 just before, during, or after the uplift, the most reasonable view 

 being that the intrusion occurred during the uplift because the same 

 great pressure could well have pushed up the molten masses during 

 the process of elevation. In some cases the Grenville appears to 

 have been pushed upward and to have been largely removed by. 

 erosion since ; in other cases the Grenville was more or less engulfed 

 by, or involved with, the molten flood as shown by the numerous 

 Grenville inclusions and the areas of mixed gneisses ; while in still 

 other cases the Grenville rocks were left practically intact as shown 

 by the large Grenville areas. These intrusives now exposed at the 

 surface were, at the time of their intrusion, deeply buried under 

 a great thickness of overlying rock material. This we know because 

 they are true plutonic rocks which could have cooled only under 

 such conditions. The vast amount of erosion since their intrusion 

 has exposed them. 



Following this great period of igneous intrusion there was a 

 time of minor igneous activity when the gabbros, in molten condi- 

 tion, were forced upward into the crust of the earth. Since the 

 gabbros now exposed at the surface are true plutonic rocks they 

 too must have been deeply buried under material which has been 

 removed by erosion. 



