I P^ NEW YORK STATE MUSEUM 



GENERAL GEOLOGIC FEATURES 



Under this heading it is proposed to briefly outline the 

 geologic history of the whole Adirondack region so that the 

 detailed study presented in this report may be made more in- 

 telligible to the reader. This outline is based largely upon the 

 admirable treatises of Prof. H. P. Gushing. 



» So far as known the oldest rocks of the Adirondacks are those 

 of Grenville (Precambric) age. They are sedimentary rocks, 

 originally shales, sandstones and limestones, which have been 

 highly metamorphosed into gneisses and crystalline limestone. 

 These rocks are of unknown but great thickness, and are widely 

 scattered throughout the Adirondacks, thus showing that the 

 whole region was under water at the time of their deposition. 



After the deposition of the Grenville sediments the region 

 was raised above the ocean level and the rocks began to decom- 

 pose and suffer erosion. Either just before, during, or after the 

 uplift, great masses of igneous rocks were intruded. The Gren- 

 ville rocks were for the most part engulfed by the intrusion so 

 that only occasional patches of them were left intact. 



After the igneous activity the rocks became thoroughly meta- 

 morphosed by being squeezed, highly folded and converted into 

 gneisses. Such changes can take place only at great depths 

 (several thousand feet) and hence we are led to the belief that 

 a vast erosion of the original land masses must have taken place. 

 This in turn signifies that the land masses must have remained 

 above sea level for an immense length of time. 



At or toward the close of this long period of erosion, igneous 

 activity of a minor character took place. The basic igneous 

 rocks erupted at this time are especially well shown in the 

 northeastern Adirondacks, where they were squeezed up 

 between joint planes in the older rocks. That these rocks are 

 much younger than the igneous rocks first mentioned is clearly 

 shown by their mode of occurrence and their general lack of 

 metamorphism. 



Toward the close of the erosion period the region of the 

 Adirondacks was nearer the sea level and of slighter relief than 

 at present. Then the whole region began to sink slowly, allow- 

 ing the sea to encroach upon the land until only an island was 

 left or probably even until the whole region was under water. 

 During the subsidence, deposition of Paleozoic sediments went 



