SURFACE GEOLOGY. 



39 



limestone, said to be nearly equal in bulk to a cube twenty feet on a 

 side. A great number of bowlders, scarcely smaller than these, are 

 mentioned in the notes of the Geological Corps, but these are sufficient to 

 show the power of the transporting agent. Smaller bowlders are some- 

 times found in immense numbers over a limited space. Near Euclid, 

 in Cuyahoga county, they are so thickly strewn over a field of several 

 acres as to resemble a large herd of sheep and cattle. Even in southern 

 Ohio they are locally very numerous. Prof. Orton mentions a belt of 

 these bowlders, two or three miles wide, between Dayton and Eaton, 

 where they are so thickly set as to make the cultivation of the soil 

 almost impossible. Here the bowlders are almost exclusively Canadian 

 metamorphic rocks, among which a granite, with rose-colored felspar, a 

 gray gneiss, diorite, and silicious slate, are the predominating varieties. 

 Along the highlands of the divide we occasionally see blocks of consider- 

 able size, which have been torn from some neighboring ledge, and among 

 the smaller rounded bowlders found on and south of the divide, a large 

 number are derived from indigenous rocks, but the greater part of the 

 larger, bowlders strewed over the surface are of foreign origin. 



Very few of the surface bowlders show any striation or planing, such 

 as is seen in those of the bowlder clay. This is a distinction that has 

 an important meaning; for so large a number of the bowlders in the 

 glacial clay (where they have been transported by glaciers) are planed 

 and scratched, that the absence of such markings from the surface bowl- 

 ders is pretty good evidence that they have had a different experience. 

 We may, therefore, conclude that the. striated bowlders have been trans- 

 ported beneath glaciers, and that the 

 rounded and unscratched bowlders 

 have not. With the great limestone 

 bowlder of Huron county are many 

 of smaller size, which are very much 

 scratched and worn. These rest on 

 the surface of the Huron shale, and 

 we can hardly doubt that they have 

 been transported and left where 

 found by a glacier moving from the 

 north. In the vicinity of these lime- 

 stone bowlders a spherical concretion 

 from the Huron shale was found, of 

 which one side is planed off as 

 smoothly as it could be done by art. Whether it was held in the glacier, 

 or in the shale, when the grinding was done, is somewhat uncertain; 



CONCRETION FROM DRIFT GROUND OFF 

 By GLACIER. 



