62 NEW YORK STATE MUSEUM 



by the great thickness of the sediments. A most conservative esti- 

 mate by geologists gives the age of the Grenville strata as no less 

 than 2=f or 30 miUion years, though of course we have no means of 

 accurately determining geologic time in terms of years. 



After the deposition of the Grenville strata came vast intrusions 

 of igneous rocks, including first the eruption of a great body of 

 anorthosite mostly in Essex count)- and including the two small 

 masses in the Blue ^lountain quadrangle, and then the still greater 

 bodies of syenite and granite, examples of which are so well 

 shown in the quadrangle. Also the whole Adirondack region was 

 raised well above sea level probably at or near the time of the erup- 

 tion of the syenite-granite series. The tilting and metamorphism 

 of the Grenville strata were most likely largely concomitant with 

 the great igneous intrusions. 



The great Precambrian land mass underwent profoiuid erosion 

 for some milhons of years at least, extending through later Pre- 

 cambrian time and even into the early Paleozoic, as shown by the 

 facts that the oldest rocks deposited upon the Precambrian are of 

 late Cambrian age, and that the Precambrian rocks immediately 

 below the Cambrian exhibit textures and structures which could 

 have developed no less than some thousands of feet below the 

 earth's surface. 



Following the great intrusions and during the time of erosion 

 above referred to came the minor intrusions of gabbro and diabase. 

 The gabbro is definitely known to be much older than the diabase, 

 the fine-grained texture of the former pro\-ing it to have cooled 

 comparatively near the surface of the earth either in late Pre- 

 cambrian or ver\- early Paleozoic time. 



Paleozoic History 



By late Cambrian time the profound erosion above mentioned had 

 worn down the whole Adirondack region to the condition of a more 

 or less well-developed peneplain. This we know because late Cam- 

 brian strata (particularly the Potsdam sandstone), which are the 

 oldest to have been deposited upon the Precambrian. everv-where 

 rest upon a peneplain siu-face of the Precambrian. 



There is no evidence that Paleozoic strata were ever deposited 

 over the area of the quadrangle, though late Cambrian or Ordovician 

 strata now almost completely surround the whole Adirondack region. 

 These Paleozoic rocks formerly mantled all but the central Adiron- 

 dacks. A number of erosion remnants of the Paleozoic rock cover 



