DRIFT DEPOSITS OF ILLINOIS. 19 



Specimens of native copper have been found in this portion of 

 the State, but their presence there may be due to human 

 agencies, as it is Avell known that copper wa\s an article of com- 

 merce among the primeval races of the Mississippi valley. 



The conditions under which the drift deposits were accumu- 

 lated, and the agencies employed in their deposition, are still 

 unsettled questions, and the complex character of the phenomena 

 presented affords a wide field for speculation. 



It has been generally conceded that a colder climate than that 

 of the present time prevailed over the northern temperate zone 

 when the boulder clays were deposited, and that ice was an im- 

 portant factor in the transportation of the foreign material 

 contained therein. 



Two rival theories, known respectively as the "glacial" and 

 the ''ice berg" theories, have been advanced to explain the 

 origin of the drift, both of which have found able advocates to 

 urge their respective claims. The latter theory attributes the 

 accumulation of the drift and the foreign materials occurring 

 therein to floating ice and water currents during a period of 

 submergence, when the entire surface over which the drift extends 

 was beneath the water. 



The ''glacial theory," on the other hand, attributes the ac- 

 cumulation of the boulder clays to the action of land ice, and 

 is based upon the assumed conditions of an arctic climate, and 

 a vast continental "glacier" or "ice cap," that was supposed 

 to have covered all the North American continent north of the 

 fortieth parallel of latitude. 



Prof. Dana, in his Manual of Geology, 2d ed., page 544, gives 

 the following as the conditions supposed to prevail over the 

 northern portion of the continent during the ''glacial period": 



"During a glacial epoch of the kind here supposed, the whole 

 northern portion of the continent down to the southern limit of 

 the Drift, would have been covered by a vast and almost unin- 

 terrupted glacier. This is now the case in North America with 

 the return of nearly every winter. But the depth, instead of 

 being as now, but a few feet, and that mostly of snow, must 

 have been, judging from the height on the hills to which the 

 striie extend, at least 4,000 to 5,000 feet, and the material, as 

 in all such thick accumulations, would have been ice, and above 



