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 Pele 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- 



