242 GEOLOGY OF ILLINOIS. 



consists principally of white and black oak and hickories, with only a small 

 proportion, though a considerable variety, of other species. The bottoms sup- 

 port a dense growth of oaks, white and black walnut, mulberry, elm, hack- 

 berry, etc., with not unfrequent groves of sugar-maple. 



Alluvium. The alluvial deposits of the bottoms, composed of the broken-up 

 materials of all the older beds which have been worn away in the excavation of 

 the valleys, together with the portions which are continually brought down by 

 the small tributaries, cover considerable surfaces, but have nowhere accumulated 

 to any great depth. 



Loess. The marly and sandy clays of the Loess, a lake deposit made before 

 the formation of the present soil, are not very thickly developed in this county, 

 though they include the brown clay subsoil which underlies almost the entire 

 surface. The only shell-bearing clay observed, though it is doubtless common 

 in the prairie sloughs, is about two miles southeast of Fairmount. The black 

 soil is here from one to two feet thick, and is underlaid by a light brown, tena- 

 cious clay, filled with the calcareous shells of Lymnea, Phi/sa, Planorbis, Sphse- 

 rium, etc. In some portions, these have decomposed, and we have white, marly 

 lumps and streaks which are characteristic of beds of this formation. At this 

 locality, the partially decayed skeleton of a Mastodon was found, in September, 

 1868. The bones were lying partly upon, partly imbedded in, this marly clay, 

 the tip of one of the tusks being within thirteen inches of the surface. The 

 slough had been mostly drained, of late years, the air had permeated the bed and 

 pretty thoroughly decayed the bones, which were doubtless in good preservation 

 so long as they were constantly covered with water. The parts were promiscu- 

 ously mingled, showing that the animal had not been left to decay undisturbed. 

 Marks of gnawing upon a few of the bones, give reason to suppose that the 

 water in which the animal lay was so shallow as to give access to wolves or 

 other carniverous animals. The fragments are now in the possession of the 

 Chicago Academy of Science. 



I am informed that, in the early settlement of the county, the bones of these 

 animals were quite common in the sloughs of this region, and even at the pre- 

 sent day the discovery of isolated fragments is no rare occurrence. It seems 

 probable that a little careful searching, in such localities, would secure some 

 still perfect skeletons. It is evident that these enormous animals roamed in 

 considerable numbers over the prairies at no very remote period. 



These beds of Loess are everywhere underlaid by the 



Boulder Drift. The deposits of this age form extensive beds, in the north- 

 ern part of this county. They have been penetrated to the depth of one hun- 

 dred and fifty feet, near the north line of the county, where they compose the 

 dividing ridge between the waters of the Big Vermilion and those of the Iro- 

 quois. Along both sides of the Middle and North forks of the Big Vermilion, 



