THE ISLANDS. 199 
tered a formidable barrier in the strata of limestone of which that arch 
is mainly composed. Hence this portion of the lake basin was less 
deeply excavated, and the most prominent or the most resistant masses 
of limestone have been left in relief, and now project above the surface 
of the Lake. It is probable, also, that the channels between the islands 
are in part due to surface erosion, for we have evidence that all the 
region about the islands was for a long period entirely above drainage. 
This is proven not only by the deeply excavated channels of all the 
streams which flow into the Lake, as Grand River, the Cuyahoga, Black 
River, the Huron, Portage, Maumee, and so on. All these streams now 
enter the Lake from one hundred to two hundred feet above their ancient 
beds, and when they flowed in their now deeply buried rocky channels 
Lake Erie had no existence as a lake, but was a valley traversed by De- 
troit River, which flowed north of Point Pelé Island at least two hundred 
feet below the present lake level, and received the streams I have men- 
tioned as its tributaries. We have other evidence that the country 
about the islands was once all dry land in the caves upon those islands, 
which were ancient subterranean water-courses, and are excavated con- 
siderably below the lake surface. 
SOIL AND VEGETATION. 
In most parts of the islands the rocks of which they are composed are 
covered with a greater or less thickness of Drift clay. This, when ex- 
posed to the air, is brown, or chocolate color, from the oxidation of its 
contained iron, and, like much of the bowlder clay on the main land, is 
filled with minute fragments of the rocks which have been excavated 
to form the lake basin, mainly Huron and Erie shale. With these are 
pebbles—rarely bowlders—of crystalline rock, evidently brought from 
the north. The clay also contains great numbers of fossils plainly de- 
rived from the Hamilton rocks. The most abundant of these is the 
Spirifera mucronata, generally worn and rounded, as though transported 
some distance from its place of origin. In a few localities, as in the 
westerly side of Put-in-Bay Island, there are heavy masses of gravel and 
bowlders, mostly of remote origin, and which, perhaps, deserve to be con- 
sidered as moraines. 
The soil of the islands is partly derived from the disintegration of the 
underlying rocks, and partly from the Drift clay. It is, therefore, highly 
charged with lime, and has proved to be so well adapted to the culture of 
the grape that nearly all the cultivated portions are laid out in vine- 
yards. The success of the grape culture on the islands has also been de- 
