408 C. K. LEITH 



i. The basic gneissic series appears in the western tier of town- 

 ships of the county, with the exception of Clinton, Peru, and Ausable. 

 The gneisses comprise several varieties, varying widely in texture and 

 composition. 



2. The series of schists and gneisses, with crystalline limestone, 

 occurs only in Black Brook township. The limestone is coarsely 

 crystalline, and much of it is quite pure, but very often it contains 

 much green pyroxene. 



3. The gabbros occur in three areas, which are outliers of the 

 main gabbro massive of the Adirondacks. One is in Ausable town- 

 ship, extending north and west of Keeseville, and is the direct pro- 

 longation northeastward of a great gabbro ridge, which comes up to 

 Keeseville from the southwest. The second forms Rand's Hill, in 

 Beekmantown and Altona townships, twenty miles north of the first 

 one. The third area forms the Catamount Mountain ridge, in south- 

 western Black Brook township. 



After the intrusion of the gabbro, and prior to the Potsdam 

 deposition, the region was subjected to intense metamorphism, resulting 

 in the foliation and granulation of the rocks, with or without sub- 

 sequent recrystallization. 



Cushing 1 maps the boundary between the Potsdam and pre- 

 Cambrian rocks north of the Adirondacks, from the line between 

 Clinton and Franklin counties, west across Franklin county into St. 

 Lawrence county, to a few miles west of Potsdam. 



The sequence of rocks in the region is believed to be as follows: 



1. A series of gneisses of great variety of structure and composi- 

 tion, in which all original structures are lost, of igneous origin, and in 

 part at least of Archean age. They seem to grade into the basic 

 gabbros of the region; at least the gabbros present phases not to be 

 distinguished from the gneisses. 



2. The Grenville series (Oswegatchie series) comprising quartzose 

 gneisses and schists, quartz-feldspar-biotite gneisses, dioritic, and gab- 

 broic gneisses, and occasional bands of coarsely crystalline limestone. 

 These rocks are accompanied by belts of gneiss, similar to the older 

 gneiss, which seem to be interstratified with the other rocks of this series, 

 but whose relationships are doubtful. The gneiss of the Grenville series 



1 Report on the boundary between Potsdam and pre -Cambrian rocks north of the 

 Adirondacks, by H. P. Cushing: Sixteenth Ann, Rep. N. Y. State Museum, 1898, 

 pp. 1-27. With sketch map. 



