GKOIvOGY IvA SALLE COUNTY. 



3 



is the chief ingredient, calcareous; and those of which 

 clav is the largest factor, argillaceous rocks. 



Slates are rocks splitting into thin, smooth plates; 

 shales, those which split in very thin, irreg-ular pieces. 

 Shales are g'enerally argillaceous, but arenaceous 

 shales are not uncommon. Limestones sometimes 

 assume this form, but generally are coarse and roug'h 

 in comparison to the sandstones. 



Rocks are divided into formations or groups which 

 are distinguished from each other by the fossils they 

 contain. These g-roups are called ag-es, and are subdi- 

 vided into eras, these into periods, and these into 

 epochs. The following- table, founded on that g-iven in 

 Dana's Manual of Geology, IVth Ed., copyright 1894, 

 imprint American Book Compan3^ 1895, pp. 410-11, 

 will give the reader a better idea of the relative posi- 

 tion and relations of these groups than would many 

 pages of description. 



Table on pages 6 and 7. 



