GEOLOGY OF LA SALLE COUNTY. 



33 



15.37, alumina, 9.13, sesquioxide of iron, 2.25. 'They 

 do not seem to have been formed by the entombment of 

 vast masses of organic remains, for of these there are 

 few traces in them, and corals and corallines are most 

 conspicuously absent. Were they formed from the 

 debris of pre-existing- limestones? If so they were 

 grpund to powder. Or is a part of our calcareous rock 

 a chemical product, the result of the action of carbonic 

 acid gas on waters charg*ed with sediments? It seems 

 to us that this may have been the source of a part of 

 these rocks, and we believe was. Tha,t all calcareous 

 rocks have been formed through organic ag-encies we do 

 not think a necessar}^ corollary from our knowledge of 

 the processes now in operation, and it seems to us 

 an unnecessary and unreasonable conclusion. That 

 there was much cla}^ and other impurities mixed with 

 it is self-evident, and this would seem to imply an off- 

 shore deposit and the almost total absence of fossils, 

 either an open and surf-washed coast, or a sea deficient 

 in material to supply food to a varied and numerous 

 population. 



The St. Peters bears in places traces of strong 

 and opposing currents, and an uneven bottom with 

 slopes of 15 ^ to 25 ^ , as if the bed in some places had 

 been formed at the mouths of strong flowing but 

 medium-sized streams. The absence of fossils may be 

 explained in several ways. The streams bearing the 

 sand to the sea mav have been cold, turbid and desti- 

 tute of food material, the sand may have been fur- 

 nished b}^ the wear of a shore washed by a cold 

 and storm}^ sea; or it may have been swept by cold cur- 

 rents which kept the temperature too low to make it 

 the favorite haunt of the tribes peopling the deep. 



The Trenton limestone was evidentl}^ deposited 

 under conditions very favorable to the development of 



