50 Ice Marls in North Wales. [Jan., 



that the vast American lakes are to some extent a- proof of it. On 

 the other hand Sir Roderick Murchison. Sir Charles Lyell, and 

 Mr. Dawson, all geologists of the greatest eminence, maintain the 

 " iceberg " theory. Exactly the same difference of opinion occurs 

 as to many other countries, such as North Russia, Finland, and 

 even Scotland, but we will now consider America only, because I 

 wish to state one difficulty which I cannot find alluded to in all 

 that has been written on the subject in this country. The iceberg 

 theory supposes that all the lake regions of North America were 

 about a thousand feet under the sea at a very recent period, that 

 the country was then ground and striated by icebergs, and has since 

 risen to its present level. Now the great lakes, Michigan and 

 Huron, are a thousand feet deep, their bottoms being about four 

 hundred feet below the sea-level. When the land rose up these 

 vast basins must have been full of salt water. vVhat has become of 

 it ? No doubt it would soon run off at the surface, and be replaced 

 by fresh, but as a mere physical problem, would all the salt water 

 from a thousand feet deep be earned off by the influx and efflux of 

 fresh water ? Has water ever been brought up from the bottom 

 of these lakes, and is it as fresh as that of the surface '? * 



But even if no trace is or ought to be found of the salt-water 

 lakes that must so recently have existed, a difficulty of a totally 

 different nature arises. These lakes and all the lakes and rivers 

 north of them to the Arctic ocean now contain great abundance 

 and variety of fresh-water fishes, and among them are many found 

 in the lakes only and some entirely confined to single lakes. There 

 are about twenty-two of these American lake-fishes described by 

 ichthyologists, most of them quite distinct and well-marked species, 

 found nowhere else in the world. About twelve are confined to 

 the group of the Great lakes, and there is one distinct genus of 

 the perch family (established by Cuvier) which has never been 

 found except in Lake Huron. Now the glacial epoch is post- 

 pliocene ; that is, it is within the period of existing species. The 

 mollusca were all identical with those now living ; the vertebrates 

 have been changed a little, but chiefly by the extinction of some 

 species. How then are we to explain the occurrence of so many 

 peculiar species and one peculiar genus in fresh-water, lakes the 

 whole district around which was so recently under the sea ? It 

 may be said that the same difficulty affects the glacier theory, for 

 if that be true, the lakes were only made by the ice and were not in 

 existence till it left them. To this it may be answered that the 

 country round the lakes in every direction was in existence though 

 the lakes were not, and we need not suppose the whole land to have 

 been covered with ice at once. It probably took different directions 



* I am informed by an eminent physicist, that by the process of diffusion the 

 ■whole of the salt water won Id no doubt in time be carried off. 



