I30 NEW YORK STATE MUSEUM 



we owe most of our knowledge concerning the subject. 1 Professor 

 Smyth has worked mainly in the outlying sections, including cen- 

 tral and western St Lawrence county and eastern Lewis and 

 Jefferson counties, but the salient facts of structure and strati- 

 graphy he has brought to light apply as well to the region under 

 discussion. 



The rocks which have widespread development comprise crystal- 

 line limestones, schists, gneisses and a series of intrusives ranging 

 from granite to basic varieties represented by the gabbros. They 

 are lithologically analogous to the prevailing rock types that are 

 described in connection with the mining districts of Clinton and 

 Essex counties and in some cases no doubt can be correlated as 

 parts of the same geologic formations, though it is not to be inferred 

 that they are strictly equivalent as to time. All are older than 

 the most ancient of the fossiliferous strata in the region, the Pots- 

 dam sandstone, and underlie the latter unconformably. 



The Grenville limestones and their associated schists (called 

 the Oswegatchie series by Professor Smyth) are relatively less 

 important in the interior than in the western part of St Lawrence 

 county. A belt of these rocks traverses the town of Pitcairn and 

 extends across the line into Jefferson county, with a length of 15 

 miles from northeast to southwest. There are good exposures of 

 the limestone at Harris ville and on the east side of Bonaparte 

 lake, in the middle portion of the belt. This is the most easterly 

 of the larger areas, as elsewhere in the interior the rocks occur in 

 isolated patches of no great size. The limestone is always thor- 

 oughly crystalline; the schists belong to the hornblendic, micaceous, 

 pyroxenic or quartzose types so characteristic of the Grenville 

 series throughout the Adirondacks. 



Among the gneisses which occupy most of the area in the vicinity 

 of the mines, there is great variety. Some are closely related to the 

 igneous rocks and have been demonstrated to be in part simply 

 gneissoid phases of the latter. t On the south side of the limestone 

 belt referred to above, syenitic gneiss grading into massive syenite 

 is exposed in force underlying an area estimated at 75 square miles. 

 It is clearly igneous and later than the limestone. In association 

 with it occurs a more acid hornblende gneiss which seems to belong 

 to the same intrusive mass, since there is a gradual transition 

 across the contact. On the north side of the limestone belt, 



'For the results of Professor Smyth's work, consult the annual reports of 

 the New York State Museum for 1893, 1895, 1897, 1898 and 1899. 



