LESSONS IN GEOLOGY. 367 



its famous rapids, gorge, and cataract. Some think the old out- 

 let of the Ontario valley was by the Mohawk and Hudson river 

 valleys, and that the St. Lawrence flows in a recent channel, at 

 least for a portion of its course. Lakes Huron and Superior are 

 supposed to have had their outlet by the Ontario valley, but 

 many suppose Lake Michigan flowed toward the Gulf. 



The glacier as it advanced southward drove before it all the 

 plant and animal life of the region, carried away all the soil, and 

 in many respects changed and modified the surface features of the 

 country. Then retreating, it deposited material for a new soil, 

 obliterated many old channels and changed many others, formed 

 ridges, hills, and lakes, in fact changing the whole face of the 

 country. 



The cause of the climate of the glacial period is one of the 

 questions which geologists have not satisfactorily answered. 

 The elevation of northern regions with the advance of the ice, 

 and their depression as the ice retreated, seems to connect the 

 events in the relation of cause and effect. Such elevation might 

 have closed Bering strait, and shut the Gulf stream from the 

 Arctic ocean, and yet it does not seem to geologists generally as 

 if such elevation was adequate to the effect. Some have at- 

 tempted to find a cause in the changes in the eccentricity of the 

 earth's orbit, combined with other astronomical causes, but this 

 idea does not appear satisfactory. Others have supposed a de- 

 pression of the Isthmus of Panama, allowing the Gulf stream to 

 pass into the Pacific ocean, as an adequate cause; but there is 

 no evidence of such depression during glacial times, and nothing 

 to show that it would have affected the climate if there had been. 

 Other theories have been advanced, but have not been received 

 with favor. At present it can only be said that most geologists 

 acknowledge elevation as a cause, but think that some other as 

 yet unexplained cause must have acted in connection with eleva- 

 tion to have produced the results. 



When did the glacial period begin, how long did it continue, 

 and how many years has it been since the ice retreated? are 

 questions often asked of the geologist. No very definite answer 



