ENGLACIAL DRIFT OF THE MISSISSIPPI BASIN. 57 



a slaty matrix through which are scattered rather distantly 

 pebbles of granitic, c]uartzitic and othpr crystalline rocks. This 

 is one of the forms of the " basal conglomerate " of Irving. 

 Other varieties of this " basal conglomerate " are present. In 

 addition to these very peculiar rocks, a quartzite of a very light 

 greenish semi - translucent hue has a wide distribution along the 

 tract. It is readily distinguishable from the numerous other 

 quartzites of the drift of the interior. Some years since, on 

 returning from my first field examination of a portion of this 

 belt, I sent a typical series of chips from the characteristic 

 erratics to Professor Irving, who had recently returned from 

 the study of the original Huronian region. He returned a suite 

 of chippings that matched them perfectly throughout, all of 

 which were taken in situ in the region north of Lake Huron. 



Among the boulders of the belt are occasionally found spec- 

 imens of impure limestone or of limy sandstone that might per- 

 haps be referred doubtfully to some member of the paleozoic 

 series ; but on the other hand, might with equal or greater prob- 

 ability perhaps be referred to the similar rocks of the Huronian 

 series. These are quite rare, never forming, so far as my 

 observations go, as much as one per cent, of the series. 

 In the several definite enumerations made to determine 

 the percentage of the doubtful specimens, the result never 

 exceeded a fraction of one per cent. In the most extensive 

 enumeration the result was about one - half of one per cent. 

 Aside from these doubtful specimens there are practicall}' no 

 boulders in the belts that can be referred to any of the pale- 

 ozoic rocks that intervene in the 500 miles between the parent 

 series north of Lake Huron and the tract over which the bould- 

 ers are now strewn. Occasionally there may be seen erratics 

 from the paleozoic series at or near the surface, but they are not 

 usually so disposed on the surface as to appear to be true mem- 

 bers of the superficial boulder tract. There is, therefore, the 

 amplest ground for the assertion that these boulder tracts are of 

 distant derivation, and that they are essentially uncommingled 

 with derivatives from the intermediate region. 



