134 GEOLOGY OF ILLINOIS. 



The Silurian fauna, disclosed in the geological horizon represented by 

 these Dixon formations, was truly wonderful. The soft mud of these 

 Silurian seas became the sarcophagi of extinct species and generations. 

 We tread reverently among these old stones, marked with forms of life 

 now fossilized ; for a great chapter of the history of the earth of the 

 story of creation lies half revealed before us. The entombed relics of 

 millions of years cycles in which man had no part Sibyline mysteries, 

 almost too great for the finite mind to grasp the story of undefinable 

 epochs, written by the infinite finger of the Creator, in strong traces 

 these and kindred thoughts come over us, when gathering the fossils. 

 No wonder Shakspeare could find "sermons in stones." The stones are 

 full of sermons ; full of an inspired revelation j they are the great Bible 

 of Creation the Stone Book, whose solid leaves are pictured over with 

 sublime truths. 



jSu rfa ce Geolog y . 



The surface geology of Lee county is also interesting. The drift beds, 

 or gravel banks the boulders or lost rocks the clays and the sand 

 the alluvial deposits of the river and the swamps these form instructive 

 chapters in a subsequent ancient history of the county. 



Alluvial Deposits. Kock river spreads out into a bottom land of limi- 

 ted extent > below Grand DeTour. This bottom is composed partly of 

 the Black river alluvium characteristic of river bottoms generally, and 

 partly of banks and ridges of river sand $ but before reaching Dixon 

 the rocky bluffs on either side have drawn close to the river's shore, and 

 for several miles below Dixon no alluvial deposits exist, except the shift- 

 ing sand bars and gravel beds in the stream. Before leaving the county, 

 the bottom again spreads out, and occasional small flat expanses are 

 covered by crumbling, marly sands and clays more recent than the true 

 drift. Even the extensive gravel beds worked by the railroad company, 

 just below Nelson Depot, are river gravel beds belonging to this division 

 of the Quaternary system. 



The common prairie soil covering the county, composed largely of 

 humus and the vegetable mold, left . by the successive growths and 

 decays of the prairie grasses, of course belongs to these recent deposits 

 and is found all over the county. But the most marked of the recent 

 deposits to be found in the county, are the swampy lands of the Green 

 river bottoms. The struggle between water and land over these affords 

 one of the finest illustrations of the origin and formation of the prai- 

 ries to be met with in this part of the State. The land can almost be 

 seen slowly encroaching upon the miry waters, and a real prairie taking 

 the place of a water-logged swamp. 



