THIS NAUTILUS. 48 



II. 



Assuming, then, that the Atlantic fauna, in its inception, was 

 derived 1'roin a very early immigration from the west, there has been 

 abundance of time for it to become specifically differentiated. Un- 

 less the unanimous opinion of the geologists of this country is en- 

 tirely wrong, it is clear that whatever remnants of this ancient fauna 

 were left along the course of this ancient track of migration towards 

 the east in the region of the Great Lakes were wiped out absolutely 

 by the invasion of the ice during the Glacial Period. Whatever 

 may be said in regard to there being any geological evidence of an 

 unglaciated area in north central British America, there would seem 

 to be no doubt but that the region of the Great Lakes was the very 

 center of the destruction wrought by the invasion of the ice. As 

 has already been said, the entire system of drainage was absolutely 

 changed. The old system was wiped out and a new and radically 

 different one established. The Great Lakes themselves are entirely 

 the result of changes in the earth's surface, wrought by the invasion 

 and subsequent retreat of the ice. It is stated by Grabau that at 

 Detroit the present surface is 180 feet above the pre-glacial surface, 

 and that the ancient bed of the pre-glacial Cuyahoga at Cleveland is 

 400 feet below the bed of the present river, and, as has already been 

 stated, the present bed of the Ohio is 150 feet above that of its 

 ancient predecessor. According to Taylor (4) the front of the re- 

 treating ice-cap at Toledo, Detroit and Port Huron stood in two 

 hundred feet of water. There is no part of the present area occu- 

 pied by the Great Lakes and their tributaries that was not included 

 within this area of glacial destruction. We may assume, therefore, 

 that whatever fauna was in existence prior to the advent of the ice 

 was wiped out absolutely from this region. This being assumed, the 

 present extension of the Atlantic fauna to the northwest must be 

 looked for in the various systems of temporary drainage that were 

 established in the post-glacial times prior to the final establishment 

 of the existing St. Lawrence system. There can be no doubt but 

 that here and there in the Atlantic region, north of the glaciated 

 area, there were places in which the remnants of the ancient fauna 

 were preserved, and that, from these harbors of refuge, upon the 

 retreat of the ice, the Unionidze were able to re-people the barren 

 waters of the new land. 



The ice in the lower lake region retreated towards the north and 



