34 



GEOLOGY OF LA SALLE COUNTY. 



both animal and veg'etable (marine forms) of life, and 

 the same is true of a part of the coal measures. 



We must not forg-et, however, to observe another 

 fact relating- to these strata. The formation of the 

 Trenton beds did not beg-in as soon as that of the St. 

 Peters ended. The evidence is conclusive that the St. 

 Peters was for a time the surface, the dry laiq^, 

 and was worn and channeled by the action of air and 

 water, and the Trenton was laid down on this roug-h- 

 ened surface, sometimes very roug^h. 



Farther north the Trenton is composed of two 

 members, if not three, the Galena limestone, the lead- 

 bearing- rock ot Northwest Illinois, a thick bedded, 

 rather hard, buff mag-nesian limestone or dolomite, 

 and lying' above it a shaly limestone, rich in fossils, the 

 Hudson River or Cincinnati shales. This last seems to 

 be wanting- here, as are also many of the fossils which 

 characterize it. 



The carbonic area was e\'ldently one of constant 

 chang-es. The fossils are, except the veg-etable, larg-ely 

 marine; hence, we are sure that much of it was at 

 times beneath the sea, or was covered by broad, per- 

 haps shallow lag-oons, to which the sea had easy access. 

 There was an exuberance of molluscan life, a consid- 

 erable number of species and a multitude of indi- 

 viduals, such a host that often g-reat masses of rock 

 are almost made up of the remains of a sing-le species. 



The plants were of few families but many g-enera, 

 and the number of individuals g-reat. But the indica- 

 tions are that it was, when at its best, no such exu- 

 berant g-rowth as today clothes 'the deltas of the 

 Gang-es, and the hig-her parts of the Amazon basin in 

 robes of g'lory and beauty. Larg-e trees there were, 80 

 to 100 feet in heigfhth and two to three feet in diameter, 

 but they did not branch widely, and these Goliaths of 



