Great Lakes, and the Valley of the Mississippi. 



~-7 



have sometimes seen tliem, a thousand or more boulders od a 

 few acres, resting on the Erie clays and looking in the distance 

 like flocks of sheep, — or dropping here and there a stone and 

 floating on east or west till wholly dissipated. 



These boulders include representatives of nearly all the 

 rocks of the Lake Superior country, conspicuous among which 

 are granites with rose-colored orthoclase, gray gneiss, and dio- 

 rites, all characteristic of the Laurentian series ; horn b lend ic 

 rocks, massive or schistose, and dark greenish or bluish silieious 

 slates, probably from the Huronian ; dolorites and masses of 

 native copper, apparently from the Keweenaw Point copper 



region. 



In the Drift gravels I have found pebbles and small boul- 

 ders of nearly all the paleozoic rocks of the lake basin, con- 

 taining their characteristic fossils, viz. : The Calciferous Sand- 

 rock with Maclurea, Trenton and Hudson with Arribonychia 

 radiata, Cyrtolites omcctus, Medina with Pleurotomaria /<'/<>/■< a, 

 Corniterous with Conocardium trigonale, Atrypa reticularis, 

 Favosites polymorpha, Hamilton with Splr'ifer mucronatux, etc. 



The granite boulders are often of large size, sometimes six 

 feet and more in diameter, and generally rounded. 



The largest transported blocks I have seen are the more or 

 less angular masses of corniterous limestone mentioned on a 

 preceding page. 



Along the southern margin of the Drift area, especially on 

 the slopes of the highlands of Northern Ohio, the Drift sands 

 and gravels are of considerable thickness, forming hills of 

 100 feet or more in height, generally stratified, but often 

 without any visible arrangement. 'J hese deposits are \, IV 

 unevenly distributed, with a rolling surface frequently forraiug 

 local basins, which hold the little lakelets or sphagnous marsh- 

 es so characteristic of the region referred to. These an- the 

 beds to which I have alluded as constituting, in the opinion 

 of some geologists, a great glacial moraine, but from the facl 

 that they are locally stratified, and overlie the older blue clays, 



