NORTH AMERICAN LAKES. 159 



from 600 to 900 feet thick, along this whole extent, 

 inclined at a very slight angle. 



Lake Mareotis. — It is well known that this ancient 

 lake of Egypt, together with the canal which con- 

 nected it with the Canopic arm of the Nile, has 

 been filled with mud and is become dry. Herodo- 

 tus observes, " that the country round Memphis 

 seemed formerly to have been an arm of the sea 

 gradually filled by the Nile, in the same manner as 

 the Meander, Achelous, and other streams had form- 

 ed deltas. Egypt, therefore," he says, " like the 

 Red Sea, was once a long narrow bay, and both 

 gulfs were separated by a small neck of land. If 

 the Nile," he adds, " should by any means have an 

 issue into the Arabian Gulf, it might choke it up 

 with earth in 20,000, or even, perhaps, in 10,000 

 years ; and why may not the Nile have filled with 

 mud a still greater gulf in the space of time which 

 has passed before our age ]" 



North American Lakes. — There are numerous 

 facts to show that these lakes occupy much less 

 space than they once did, and that they are con- 

 tinually diminishing in size. Parallel to the south- 

 ern shore of Lake Erie, and from four to eight miles 

 distant from it, is a ridge of land, composed of sand, 

 gravel, and rolled pebbles, such as now make the 

 shore of the lake. This ridge is elevated from 140 

 to 200 feet above the lake, and it is now generally 

 admitted to have once formed the lake shore. 

 Wherever wells have been dug or excavations 

 made in this ridge, fragments of decayed wood, 

 bark, and often branches and trunks of trees, are 

 found deeply imbedded in the soil, together with 

 such species of shells as are now met with in the 

 lake. Such ridges also exist at a considerable dis- 

 tance from the shores of Lake Erie, Superior, Mich- 

 igan, and Huron, showing that their waters once 

 occupied a much higher level than they do at present. 



It has been ascertained by Dr. Bigsby, that the 



