GEOLOGICAL SURVEY OF THE STATE OF NEW YORK. 



(GEOLOGICAL MAP.) 



REPORT ON THE BOUNDARY BETWEEN THE POTSDAM 

 AND PRE-CAMBRIAN ROCKS NORTH OF THE 



ADIRONDACKS. 



By H. P. Cushing. 



Contents. Introduction, p. 5 ; Topography, p. 6 ; Glacial Deposits, p. 7 ; Se- 

 quence of Geologic Events in the Adirondacks, p. 8 ; Gneisses, p. 9 ; Grenville (Oswe- 

 gatchie) series, p. 10; Anorthosite intrusion, p. 11 ; Later Gabbros,^. 11; Granite, 

 p. 12; Dynamic Metamorphism of the Region, p. 12; Pre-cambrian Dikes, p. 12; 

 Palaeozoic Pocks, p. 13 ; Post-Utica uplift, y. 13 ; Post-Utica Dikes, p. 14; Faults, 

 p. 14; Local Geology; Franklin county, p. 16; Bellmont, p. 16; Burke, p. 17; 

 Malone, p. IS ; Bangor and Brandon, p. 19 ; Dickinson, p. 20 ; St. Lawrence county, 

 p. 21 ; Ilopkinton, p. 21 ; Parishville, p. 23 ; Pierrepont and Potsdam, p. 24 ; Calcif- 

 erous formation, p. 23 ; Dikes, p. 27. 



Introduction. 



The work of tracing this boundary was begun at the Clinton-Franklin 

 line, connecting with the writer's work of the previous season. Thence the 

 boundary was mapped across Franklin county and into St. Lawrence 

 to a few miles west of Potsdam. It was not known to me until after my 

 return from the field that part of the area traversed in the latter county had 

 already been studied and reported upon by Professor Smyth. As the report 

 has not, at this writing, been published, what follows must unfortunately be 

 written without reference to it. 



The country north of the Adirondacks is so heavily covered with drift 

 that outcrops of the Palaeozoic rocks are very infrequent and boundary map- 

 ping is rendered hazardous on that account. Over much of the distance the 

 best that could be done was to map the northerly limit of Pre-cambrian out- 

 crops. In Franklin county and as far west as Parishville in St. Lawrence 

 county, the mapping was a simple matter. The gneisses are so similar in 



