PRE-CAMBRIAN LITERATURE OF NORTH AMERICA 471 
pebbles and bowlders. They have little argillaceous matrix about 
the inclusions in massive conglomerates. They are not ‘“‘slate” 
conglomerates, as part of the Gowganda formation is. Their 
character is essentially that of a basal or alluvial deposit. In part 
they are well sorted and in part they are massive, but the massive 
material is composed characteristically of dark-colored graywacke, 
or of gritty or bowldery conglomerate. In no instance is there 
record of striated, glacially soled bowlders being found in Bruce 
conglomerates. Nowhere to our present knowledge is there a 
polished basement beneath them. The matrix of the Bruce con- 
glomerates is commonly dark-colored, arkosic, and graywacke-like; 
the matrix of typical Cobalt tillite is green, pale-green on weathered 
surfaces, and looks like metamorphosed clay. So clear is the 
difference in the character of certain phases of the matrices that 
members of Collins’ parties from 1914 to 1918, with some practice, 
were able to tell from a glance at typical hand specimens whether 
or not the rock was Bruce or Cobalt conglomerate. This difference 
in the character of the matrix is a genetic difference, connected 
with the glacial origin of the one and the non-glacial origin of the 
other. Those phases of the Bruce conglomerate which are very 
similar to the less characteristic phases of the Cobalt conglomerate 
conceivably may be of an obscure, glacial origin. 
It is not altogether surprising that a reviewer of the literature 
should fall into confusion. Originally, in accordance with the best 
nomenclature of the day, Coleman’ referred to the Cobalt con- 
glomerate as Lower Huronian, as is quite clear from his writings in . 
1907 and 1908; whereas the only locality of distinctly glacial 
deposits he cited is the Cobalt silver-producing district which is 
underlain by the upper, Cobalt series, not by the Bruce con- 
glomerate now known as the lower series. Furthermore, Coleman, 
himself, at one time seems to have been confused by the similarities 
between the Bruce and the Cobalt conglomerates. Indeed Pro- 
fessor Willmott? previously (1901) had written: ‘“‘The two slate 
t A. P. Coleman, “‘The Lower Huronian Ice Age,”’ Jour. Geol., Vol. XVI (1908). 
p- 149; Bull. Geol. Soc. Am., Vol. XIX (1908), p. 355; Am. Jour. Sci., Ser. 4, Vol. 
XXIII (1907), pp. 190-01. 
2 A. B. Willmott, American Geologist, Vol. XXVIII (1901), p. 19. 
