72 GEOLOGY OF OHIO. 



Some hundreds, perhaps thousands, of feet have been taken off the Lau- 

 rentian hills, and ground up and distributed over all the Drift area. 

 The gold contained in this mass has shared the fate of the associated 

 minerals, has been finely triturated, and has been carried as far as the 

 glaciers reached. As the various conglomerates contained in the Carbon- 

 iferous series have obtained their quartz pebbles from the same region 

 which supplied those of the Drift, it is almost certain that gold is con- 

 tained in all of them. As these rocks have been eroded, they may also 

 have contributed something to the large aggregate quantity of gold dis- 

 seminated through our superficial deposits. 



ORIGIN OF THE GREAT LAKES. 



The question of the origin of the great lakes is one that requires more 

 observation and study than have yet been given to it, before we can be 

 said to have solved all the problems it involves. There are, however, 

 certain facts connected with the structure of the lake basins, and some 

 deductions from these facts, which may be regarded as steps already taken 

 toward the full understanding of the subject. These facts and deductions 

 are, briefly, as follows : 



1st. Lake Superior lies in a synclinal trough, and its mode of forma- 

 tion, therefore, hardly admits of question, though its sides are deeply 

 scored with ice-marks, and its form and area may have been somewhat 

 modified by this agent. 



2d. Lake Huron, Lake Michigan, Lake Erie, and Lake Ontario are 

 excavated basins, wrought out of once continuous sheets of sedimentary 

 strata by a mechanical agent, and that ice or water, or both. 



That they have been filled with ice, and that this ice formed great 

 moving glaciers, we may consider proved. The west end of Lake Erie 

 may be said to be carved out of the Corniferous limestone by ice action; 

 as its bottom, and sides, and islands — horizontal, vertical, and even over- 

 hanging surfaces — are all furrowed by glacial grooves, which are parallel 

 with the major axis of the Lake. 



All our great lakes are probably very ancient, as, since the close of the 

 Devonian period, the area they occupy has never been submerged be- 

 neath the ocean, and their formation may have begun during the Coal 

 Measure epoch. 



The Laurentian belt, which stretches from Labrador to the Lake of the 

 Woods, and thence northward to the Arctic sea, forms the oldest known 

 portion of the earth's surface. The shores of this ancient continent, 

 then high and mountainous, were washed by the Silurian sea, where the 



