12 NEW YORK STATE MUSEUM 



GENERAL GEOLOGY 



The rocks of these three quadrangles consist of (a) crystaUine 

 rocks of Precambrian age and (b) early Paleozoic rocks of the St 

 Lawrence trough which lie upon the Precambrian rocks. The 

 Precambrian rocks are the characteristic rocks of the Adirondack 

 region, are as old as any rocks of which we have knowledge, 

 consist in part of sediments and in part of igneous rocks, and have 

 but small extent on the sheets here reported upon, occupying about 

 one-fourth of the area of the Ogdensburg sheet, and occurring in 

 the southeast corner of the Brier Hill sheet. On the Hammond 

 and Gouverneur sheets, next south, -however, they occupy most of 

 the territory. The overlying Paleozoic rocks extend solidly along 

 the St LawTence in a strip a few miles in breath, and the lower- 

 most member, the Potsdam sandstone, is also found in outlying 

 patches within the area of the crystalline rocks. The crystalline 

 rocks are much older than the Paleozoics, venerable as the latter 

 are, and in the time interval between the two sets of rocks the 

 region existed as a land area for a very long period, during which 

 time much rock material was slowly worn away from its surface. 

 It is upon this worn and somewhat irregular surface of the crystal- 

 line rocks that the Paleozoic rocks rest. They have remained com- 

 paratively undisturbed since their deposition, and still lie nearly 

 flat and unbroken, as laid down, presenting the strongest kind of 

 a contrast to the greatly deformed Precambrian rocks. 



DESCRIPTIVE GEOLOGY 



PRECAMBRIAN ROCKS 



The Precambrian rocks of northern New York comprise an old 

 series of sedimentary rocks, known as the Grenville series, which 

 are the oldest known rocks of the region, and various masses of 

 igneous rocks, all of which cut the Grenville rocks intrusively and 

 are therefore younger than they are. The older set of these intrus- 

 ives consists chiefly of granite, while a younger set consists of 

 granites, syenites, gabbros and anorthosites. Where both sets are 

 present and in contact w^ith one another, as is the case in the 

 Thousand Islands region, it is possible to class them in their appro- 

 priate groups, on the evidence of their structural relations to one 

 another. But where such contacts can not be found, such classi- 

 fication is a difficult and hazardous matter. To the older set of 



