6 BOTANY OF L,A SAL,LE COUNTY. 



farther side wolud tear off blocks of rock which would 

 be either rolled or pushed before it, frozen into it and 

 borne along- with it. Thus it would grind and plane 

 down the face of the reg-ion over which it moved, 

 tearing- out great blocks of rock from their beds, 

 crushing- and breaking- these into smaller masses and 

 grinding- the smaller frag-ments into powder. Besides 

 its grinding- action on the rocks over which it traveled 

 would form a great quantity of very fine material such 

 as would g-o to make clay. Now this is just what has 

 happened and we conclude that there must have been 

 a time when a larg-e tract of country to the north of 

 Michig-an and Wisconsin was covered by an immense 

 deposit of ice, 



We will now consider the strata beolow the coal 

 measures. We should have below the coal measures 

 some beds called the Sub Carboniferous, but they do 

 not exist; then the Devonian and we find nothing- of it; 

 below it the Silurian upper and lower, but we find no 

 upper Silurian the -Niag-ara belong-s to that and we 

 have but little of the lower Silurian. We find in a 

 few places a gray, or yellowish gray, hard, half crys- 

 talline limestone which the fossils prove to be the 

 Trenton limestone, a member of the lower Silurian 

 group. It occurrs on Covel Creek, a half a mile 

 from its mouth. About a mile and a half west of the 

 court house, Ottawa, near the east line of La Salle 

 township on the C. R. I. & P. R. R., near the villag-e 

 of Troy Grove, along- the Big- Vermillion river from 

 Lowell to Deer Park Canyon at intervals and in a few 

 pls.ces on the Fox river. Below it lies a soft, friable, 

 g-enerally lig-ht colored, often white, sandstone known 

 as the St. Peter's Sandstone, which is g-enerally rap- 

 idly wasted aw r ay by the weather. It is larg-ely com- 

 posed of pure silica, This is the upper member of the 



