64 GEOLOGY OF ILLINOIS. 



the banks of the river and the small streams by which it is intersected, of 

 sands, clays, and vegetable mould, more or less perfectly stratified, and fre- 

 quently replacing each other at short intervals. It has been formed, in part, 

 from the transported material brought down by the river current, together with 

 the vegetable and animal substances that decay upon the surface, to which is 

 added, the sands, clays, and organic matter, that is washed down upon it from 

 the neighboring hills. 



The Loess is restricted to the region adjacent to the Illinois river bluffs, and 

 attains a maximum thickness of nearly a hundred feet, but thins out gradually 

 from the bluffs towards the central portions of the county. It consists of 

 brown, and drab colored sandy, and marly clays, sometimes partially stratified, 

 and varying in color, with the variable quantities of the oxyd of iron it con- 

 tains. It is well exposed in the vicinity of Versailles, and forms the main por- 

 tion of the hills adjacent to that town, and is exposed in the cuts along the 

 Quincy and Toledo railroad, westward, nearly to Harshman Station. At La- 

 grange, the Loess and Drift formations overlie the Coal Measures, and are, by 

 measurement, one hundred and ten feet in thickness, the greater portion of 

 which may be included in the Loess. It contains here a few of the land and 

 fresh water shells, which are the most characteristic fossils of this group at 

 other points, but they are less abundant here than at Quincy, and many other 

 localities in the State. 



The Drift formation in this county presents the same general characters as 

 in the adjacent counties, and consists of unstratified clay and gravel, usually of 

 a brown or ashen gray color, containing boulders of igneous and metamorphic 

 rocks disseminated through it, but most abundant in the lower portion of the 

 deposit. As no rocks similar to these boulders are to be found within the 

 limits of this State, it is evident that a large portion of the material composing 

 this formation, has been transported from abroad, and by comparing specimens 

 of these boulders with the nearest known outcrops of similar rocks in situ, it 

 has been demonstrated that much of this material has been derived from the 

 region lying to the north of Lake Superior. 



The transportation of this Drift material has been brought about by the com- 

 bined agencies of ice and water, during a period of submergence, while the en- 

 tire area of this and several of the adjoining States was beneath the water level. 

 Icebergs, impelled by winds, or currents of water, and loaded with the detritus 

 of distant shores, were, no doubt, one of the most potent agencies in the accu- 

 mulation of the Drift, and we find, as we trace this deposit southward from 

 the Lake Superior region, that the boulders diminish in size and number, in 

 that direction, until they entirely disappear. 



When we consider the conditions under which the Drift formation has been 

 accumulated, it seems hardly possible that valuable mineral deposits could be 



