Geology of Lake Superior. 31 
The trecs were in general in good preservation, and in only 
one instance, we observed a crack in one of them, filled with 
dark red oxide of iron and pyrites. 
_ From what has been just related, it is evident that the lake 
is gradually changing its shape, by the destruction of the cliffs, 
and the consequent formation of the downs, The lighter ar- 
gillaceous particles of this disintegrated sandstone, as well as 
those brought down by the rivers, are carried further out into 
the lake ; for we observed that when the depth exceeded 7 
fathoms, the bottom was generally of mud or soft clay. 
The head, or S. W. end of Lake Superior,’is formed by a 
ridge of sand hills, slightly curved to the westward, which exe 
tends across the lake about 9 miles from the sandstone on the 
south to the older rocks of the north coast. About 15 miles 
to the S.W. these rocks close ; leaving only a passage for the ri- 
ver St. Louis, which there falls over sandstone, as we were in- 
formed by the gentlemen of the American Fur Company, who 
reside near the rapids, The sandy ridge which we have men- 
tioned, may be considered as the base of a (riangle, the apex 
ef which is the rapid, and the sides, the ridges of sandstone, 
and of the older rocks on the north and south. Through this 
triangular space or valley, the St. Louis winds among alluvial} 
islands of its own formation. 
These islands do not continue to the ridge of sand hills form- 
ing the head of the lake, for, besides this outer ridge, there ig 
another parallel to it, about a mile further to the S. W. cute 
ting off a section of the triangle, and forming a small lake or 
large pond, with about 9 feet of water. 
The southern part of this pond is filled with reeds and rush« 
es, through which another considerable river flows from the 
southward, having a common outlet with the St. Louis. The 
outlet of the St. Louis through the inner, is not opposite to 
that through the outer ridge ; but there are evident marks of 
& former outlet in the latter, which was nearly so, 
The 
