96 NEW YORK STATE MUSEUM 



HISTORICAL GEOLOGY^ 

 Precambrian History 



The oldest records of the Lake Placid quadrangle are written in 

 the rocks of the Grenville series. A most conservative estimate by 

 geologists gives the age of the Grenville strata as no less than 25 

 or 30 million years, but it must be admitted that we have no 

 means of accurately measuring geologic time in years. Since the 

 Grenville rocks are distinctly stratified, very thick (many thousands 

 of feet), and of wide areal extent not only throughout the Adiron- 

 dacks, but also in eastern Canada, we may be sure that the earliest 

 known condition of the area of the quadrangle was a sea in which 

 the Grenville sediments were accumulated layer upon layer on the 

 bottom. 



After the deposition of the Grenville strata came vast intrusions 

 of molten masses, including first the upwelling of the great body 

 of anorthosite in Essex and Franklin counties, and second the still 

 greater syenite-granite body, these two igneous series being by far 

 more extensively developed than any other rocks of the quad- 

 rangle.^ During the processes of intrusion and cooling of the 

 magmas, the anorthosite differentiated into the Marcy and White- 

 face types, and the syenite-granite split up into various types rang- 

 ing from rather basic (dioritic) phases, through quartz syenite, 

 granitic syenite, and granite to even granite porphyry. The syenite- 

 granite intrusion appears to have taken place not long (geologi- 

 cally) after the anorthosite intrusion so that the latter was still hot, 

 though probably not molten, and it was locally assimilated along 

 the borders of the invading syenite-granite magma, thus giving 

 rise to the rock called the Keene gneiss. 



The whole Adirondack region was raised well above sea level 

 most likely at or near the time of the great intrusions. There are 

 strong reasons for believing that none of the rocks were ever highly 

 folded by orogenic movements, but that the breaking up and 

 tilting of the Grenville strata resulted from the upwelling of the 

 great bodies of magma; that the metamorphism of the Grenville 



^ A treatise dealing with the geography and geological history of the 

 Adirondack region in somewhat untechnical language was prepared by 

 the writer and was published as Bulletin 193 of the State Museum under 

 the title "The Adirondack Miountains." 



2 As already suggested, possibly some gabbro now represented by amphib- 

 olite, was intruded before the anorthosite. 



