110 GEOLOGY OF ILLINOIS. 



failed to detect through Ogle county. It doubtless exists if proper ex- 

 cavations were made, but the common light-colored yellowish clay is by 

 far the most common. 



Remains of the Mastodon have been found closely connected with 

 this formation. In 1858, the tooth of one of these animals' was found 

 in a little tributary of Stillman's run. The locality is low somewhat 

 marshy. The stream has cut a channel through the black alluvium of 

 the low prairie. The tooth was washed out and lodged against a clump 

 of willows when found. It is a ponderous grinder, weighs seven and 

 one-half pounds, is covered with a black shining enamel, and is a fine 

 fossil in a high state of preservation. The fortunate finder carefully pre- 

 serves it, and cannot be induced to part with his treasure. 



Other mastodon remains doubtless exist about the marshy springs of 

 Stillman's run. 



Some years ago a large bone, supposed to be from the fore-leg of one 

 of these animals, was found two or three miles above Byron. The bank 

 of Eock river had caved down for some distance back from the stream; 

 some five feet below the surface of the high land coming up to the river, 

 and perhaps fifteen feet above ordinary water-level, the bone was found 

 sticking in the bank. The bank seems to be a sort of a modified drift, 

 made up of somewhat marly, dark-colored alluvial clay, intermixed with 

 river sand and a considerable quantity of gravel. The formation is 

 hardby alluvium, but seems to be a kind of a river drift. The fossil is 

 light, porous, and whitish in color, in a rather poor state of preserva- 

 tion. We obtained it through the courtesy of Mr. Mix, and sent it to 

 the State Geological Cabinet. 



Among the mineral substances found in the drift of this county, bits 

 of lead and pieces of pure Lake Superior copper are occasionally met 

 with. 



The Cincinnati Qroiiy>. 



This formation is but lightly developed in Ogle county. No exposed 

 out-crop, that we are aware of, exists at all. The high prairie, however, 

 east and northeast of Polo, lying between Pine creek and the Illinois 

 Central liailroad, and extending a few miles north towards Adeline, is 

 underlaid by the shales of the Cincinnati group. At several recently 

 dug wells, piles of these cream-colored and blue shales and clays at- 

 tracted our attention. They are generally struck at a depth of fifteen or 

 twenty feet, and soon crumble to pieces when exposed to the rains and 

 frosts, and other atmospheric influences. The exact thickness of this 

 group I am unable to state, but think it exceeds rather than falls below 

 twenty-five feet. The area indicated is covered by the usual light-col- 

 red, finely comminuted clays, which nearly always rest upon the rocks of 



