378 GEOLOGY OF THE SECOND DISTRICT. 



upon the river, the line of junction runs northeast, so as to pass about two miles north of La- 

 fargcville ; and when opposite this place, it turns south to the middle of Perch lake ; at this 

 point, it is nearly west of the great bend in the Indian river at Evans' mills. From Perch 

 lake, it again runs northeast to a point four miles south of Theresa falls. This irregular 

 curved line defines accurately the line of junction between the two rocks, for all that por- 

 tion of the county west of Indian river. Now upon the east side within the great bend, the 

 potsdam occupies the whole space, with the exception of that which is taken up by the pri- 

 mary. The calciferous oidy passes to the north side, just at the bend opposite the mills ; 

 occupying a small triangular space just within the bend of the river. 



I have already remarked of the district enclosed by the great bend of the Indian river, that 

 it consists of masses or ridges of sandstone, and bands of gneiss alternating with each other. 

 In the lower lands, the gneiss is exposed in consequence of the removal of the sandstone, 

 while the latter occupies principally the ridges. This disposition of the masses continues, 

 with some slight and unimportant variations, to Lewis county upon the east. Connected 

 with these beds of sandstone, is one which appears about three or four miles north of Car- 

 thage, on the Wilna road, which, for some distance, it passes over. This mass spreads out 

 to the east into low grounds, in a territory which I did not particularly examine. Again, tlie 

 road from Wilna to the Natural bridge passes over the potsdam, near the line of junction 

 with the calciferous. On the road from Wilna to Leraysville, the sandstone appears in a 

 small stream, about four miles distant from the former place. The whole rock is concealed, 

 except where the drift and soil is deeply denuded. 



I have now given the extent of this rock in Jefferson county ; and though it is probably 

 continuous over the area which I have attempted to define, still it is not possible to determine 

 its presence under the great sand-plain which lies between Leraysville and Wilna or Car- 

 thage. There are several places, however, in the region occupied by the potsdam sandstone, 

 which require a brief description. The first which I shall notice, is 



The Sandstone at French creek. 



This mass is laid bare for some distance, and the surface exposed consists of a yellowish 

 brown sandstone, smoothed by the passage of drift. The deep grooves upon the surface, 

 taken in connection with the extreme hardness of the rock, strongly favor the view that its 

 surface was exposed to passage of drift for a long time. 



Leaving the hard mass which underlies the village, and proceeding on the road to Depau- 

 ville, we come to an uplift one mile east. The exposed rock is sandy and friable, and con- 

 , tains the Lingula antiqua. This small thin shell is preserved without mineralization, and the 

 mass which contains it is perfectly arenaceous, and filled with dark brown patches, which 

 appear like infiltrations of iron, though they may have been caused by the presence of animal 

 or vegetable matter, which has combined with the ferruginous matter previously existing in the 

 rock. This arenaceous mass lies immediately below the fucoidal layers, which indicate every 



