JEFFERSON COUNTY, 



379 



vchcTC the commencement of the calciferous sandrock ; and ,we find, on proceeding farther, 

 that the calciferous soon becomes the predominant rock in tliis direction. 



The Potsdam Sandstone of TJieresa falls . 



This rock is well exposed at the falls, by an uplift which discloses some interesting facts. 

 At this place, the Indian river which comes from the south, falls about seventy feet over a 

 ledge of gneiss, limestone and serpentine, into quite a large basin below, where it is nearly 

 on a level with Black lake eighteen or twenty miles distant, and becomes beatable from the 

 foot of the lake, with the exception of a few feet fall at two or three places in Rossie. 



The uplift appears to have been caused at this place by the disruption of the primary lime- 

 stone. Here w^e find some of the strongest evidences, not only of this fact, but also of the 

 former condition and of the true character of the rock. The sandstone appears here in steep' 

 broken cliffs, and separated by masses of the primary. Under one of these fractured cliffs, 

 a few rods east of the bridge and on the south side of the road leading east, the primary lime- 

 stone is in contact with the potsdam sandstone, and here we may observe the changes which 

 have taken place. Just at the line of junction, the sandstone is consolidated and vitreous ; the 

 granules which compose the rock have lost their individuality, and are undistinguishable ; and 

 while the rock has undergone this change, it has also become porous, or rather filled with 

 small irregular cavities which have been occupied with spar. These cavities are confined to 

 the vicinity of the limestone ; and a few feet above, it is in its ordinary state. There is evi- 

 dently a change at the line of junction of the two rocks ; and from a combination of facts, I 

 am satisfied with connecting them together as cause and effect ; and so long as no other cause 

 than the limestone can be discovered, this connection appears not only rational but unavoi- 

 dable. 



Passing from Theresa falls east, we shall find the arrangement of the rocks as described ; 

 the potsdam alternating repeatedly with the primary, as in the following diagram, in which a, 

 a, a, are sandstone ridges ; b, b, gneiss and hornblende ; c, c, clay, occupying the valleys. 

 The section takes in about two miles, towards Theresa falls, from the Shirtleff ore bed. 



To the w-cst, after leaving the falls, the potsdam sandstone is less disturbed, and we find it 

 continues three or four miles. But in this direction we find one of the most common changes 



