New York State Museum 



GEOLOGY OF THE CRYSTALLINE ROCKS IN THE 

 VICINITY OF THE ST LAWRENCE RIVER 



In earlier reports and papers the writer has presented the re- 

 sults of a general reconnaissance of the western Adirondack 

 region, including under this designation the relatively low lands 

 of central and western St Lawrence county and eastern Lewis 

 and Jefferson counties, which, though topographically quite 

 different from the Adirondack region proper, can not be differ- 

 entiated from it geologically. 



The present report deals in a similar manner with the small 

 area of crystalline rocks along the St Lawrence river, which, on 

 existing maps, is cut off from the main body to the southeast by 

 a narrow snip of Potsdam sandstone. The rocks in question 

 are, of course, beneath the sandstone, continuous with the main 

 body, and must be treated as such rather than as anything (lis 

 tinct. 



The region studied embraces the whole of the town of Alexan- 

 dria and parts of Clayton and Theresa in Jefferson county, to 

 gether with portions of Rossie and Hammond in St Lawrence 

 county. Within this area are included the Thousand islands of 

 the St Lawrence river. 



As might be expected, the rooks of this region fall in with the 

 types described in former reports, though there are some inter- 

 est ing variations, and a number of exposures giving data of an 

 unusually positive nature. 



For the purpose of mapping, the rocks were divided into four 

 formations, whose character, origin, and interrelations form the 

 theme of this communication. The topography is so closely de- 

 pendent on the nature of the rocks, that it is best treated, how- 

 ever briefly, after the rocks have been described. The rock 

 classification is tentative in many respects, and the interpreta- 

 tion of the mapping involves a consideration of the conditions 

 which require, for the present, such a tentative method. 



