56 THE JOURNAL OF GEOLOGY. 



If these boulder belts were of the same nature as the average 

 boulders of the till - sheets beneath them, then the simple fact of 

 unusual aggregation might be plausibly referred to the accidents 

 of gathering and deposition. But they are very clearly distin- 

 guished from the average boulders of the till by several 

 characteristics. 



1. They are superficial. Sometimes they rest completely on 

 the surface, sometimes they are very slightly imbedded, some- 

 times half buried, sometimes they protrude but a slight portion, 

 and sometimes they are entirely concealed, but lie immediately 

 at the surface. In all cases the aggregation is distinctly super- 

 ficial. Where they are buried, the burying material is usually of 

 different texture and composition from the subjacent till, and 

 appears to be distinct in origin from it. The superficiality of the 

 tract is very obvious almost everywhere, and is especially so in 

 regions where the subjacent till is of the pebble - clay rather 

 than boulder - clay order, for the comparative absence of bould- 

 ers below emphasizes the contrast. Throughout most of the 

 region the subjacent till is not of a very bouldery type, so that 

 the distinction is generally a marked one. 



2. The boulders of the belts are almost without exception 

 derivatives from the crystalline terranes of Canada. Those of 

 the great tract especially under consideration were derived from 

 the typical Huronian rocks of the region north of Lake Huron, 

 and from granitic and gneissoid rocks referable to the Lauren- 

 tian series of the same region. These last, however, cannot be 

 sharply distinguished from the granitic rocks derived from 

 other parts of the Laurentian terrane. The Huronian rocks are 

 very easily identified because of the peculiarities of some of the 

 species. Among these the one most conspicuously characterized 

 is a quartz - and -jasper conglomerate. The matrix is usually a 

 whitish quartzite. This is studded with pebbles of typical red 

 jasper and of duller rocks of jasperoid nature, which grade 

 thence into typical quartzite pebbles. With these are mingled 

 crystalline pebbles of other varieties. Another peculiar erratic 

 comes from the " slate conglomerate " of Logan. It consists of 



