GEOLOGY. 251 



epoch, has never been invaded by the waters of the sea. What, then, 

 is there strange in the fact, that the one should have had mammoths 

 at a time when the others had none ? Thus far I do not see any reason 

 for assuming, with M. D'Archiar, two epochs of striae and furrows. 

 What is the law, either paleontological or zoological, which proves 

 that the same animals have lived everywhere at the same time and 

 epoch ? ' 



M. Martins, as will be seen, enters here upon an entirely new ground, 

 by setting aside one of the broadest principles of geology, which was 

 almost a creed. Mr. D. said that he was not prepared to meet the ques- 

 tion on this new ground, which could not fail to attract attention, and 

 will undoubtedly give rise to many interesting discussions. 



ON THE EXISTENCE OF DUNES ON THE SHORES OF THE AMERICAN 



LAKES. 



AT a meeting of the Boston Society of Natural History, March, 

 1851, Mr. Desor made some statements in relation to the existence of 

 Dunes on the shores of the upper American Lakes. He said that they 

 were peculiarly interesting as being the only ones of any consequence, 

 so far as his knowledge went, which had been noticed on the borders 

 of an inland sheet of water. On the eastern shore of Lake Michigan 

 they are several hundred feet high. On the north shore, at Point aux 

 Chiens, they are from 80 to 100 feet high, the highest being half a 

 mile from the Point. They gradually diminish in size, extending 

 along the shore to the west some six miles, until they are reduced to 

 heaps not more than 25 feet high. They present no signs of stratifi- 

 cation, nor do they contain pebbles, except a few in the depressions 

 between the hillocks, where they have been thrown by the waves in 

 severe gales. The back slope has an angle of 32. In those positions 

 where the ridges are perpendicular to the coast line, the steep side is 

 always opposite to that of the prevailing wind. This feature is also 

 noticeable at several places along the shore of Lake Superior. A pe- 

 culiarity of the dunes of Lake Michigan is, that they are often covered 

 with trees of considerable size. A white pine, growing on the top 

 of the highest ridge, was found by Mr. Desor to be eight feet in 

 circumference, showing that, at this spot, the dune has remained un- 

 changed for a considerable time. The existence of these dunes on the 

 borders of fresh water refutes the opinion hitherto held by geologists, 

 that the action of the tide is necessary for their formation, enough 

 sand drying, during the ebb, to be borne off by the winds. On the 

 other hand, as they occur only on flat coasts, where, the waters are 

 subjected to considerable motion, either from currents or from the 

 action of the winds, or both combined, it is fair to regard these as the 

 agents by which they have been formed. 



ON THE GEOGRAPHY OF CENTRAL ASIA. 



CAPT. STRACHEY, at the British Association, 1851, presented a com- 

 munication on the general configuration of Central Asia, in which he 



