26 GEOLOGY OF OHIO. 
of more distant rocks are found. Many fragments of blue limestone are 
scratched and polished on their sides, but their edges are still unworn. 
The bowlders belong almost without exception to the crystalline and 
igneous rocks that are found in situ only to the north of the great lakes. 
Specimens of northern ores—iron, copper, and lead—are sometimes, 
though rarely, met with. The occurrence of gold in the bowlder clay, 
and in the gravels derived from it, is a matter of considerable theoretical 
interest, and seems never to have attracted the attention which it well 
_ enough deserves.” 
Rolled fossils derived from the older rocks are not uncommon in the 
Erie clay—Spzrifer mucronatus from the Hamilton being the most abun- 
dant. Except these, I have never discovered any organic remains in 
the formation, though shells and timber are reported to have been found 
in it. From the fact that it is a mass of glacial detritus, it seems almost 
impossible that any shells could be contained in it, and I suspect that 
all the cases of this kind reported are errors occasioned by confounding 
the Erie clay with the overlying beds of later date. The buried timber 
found at Cleveland lies distinctly above, and not in the Hrie clay. The 
piece of wood obtained by Mr. M. C. Read, referred to in the report on 
Lake county, was a rolled fragment, and coniferous. It was thought 
by Mr. Read to be buried in the Erie clay, but it was found near the 
lake-shore, and may possibly have been deposited there by other agen- 
cies than those which spread this formation. 
From the fact that the Erie clay, with its bowlder and laminated divi- 
sions, holds the same relation to the glaciated surface with the ‘“‘bowlder ” 
and “‘ Leda” clays of eastern Canada, and the ‘‘ Champlain” and “ glacial” 
clays of the Atlantic coast, it has been regarded as their equivalent, in 
time as well as place. The proof of identity is, however, yet wanting. The 
bowlder and Leda clays of the St. Lawrence valley accumulated in a sub- 
sidence of the eastern coast, in which the waters of the Atlantic followed 
the retreating glaciers, covered and in part stratified the materials 
eround up by them; and they contain marine shells of an arctic char- 
acter. These clays do not, however, reach far enough inland to connect 
with the Erie clays of the lake-basins, and it is quite possible that they 
were not exactly synchronous. 
By Prof. Dawson the greater part of the markings which are usually 
attributed to glaciers are supposed to have been produced by icebergs, 
and it is his theory that, by a general subsidence of the continent, an 
arctic current, carrying icebergs, flowed up the St. Lawrence valley 
through the basins of the great lakes, accomplishing in great part the 
erosion which has been effected there, and passing downward to the 
