NORTH AMERICAN LAKES. 159 
from 600 to 900 feet thick, along this whole extent, 
inchned 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 
