SOURCE OF THE DETRITAL MATERIAL. 31 
on Lake Superior geology. Foster and Whitney supposed them to come 
from the friction of the ascending igneous rocks against the rocks pene- 
trated,! but they ignored the totally different natures of the porphyry of 
the pebbles and of the diabases forming the greater part of the series. Be- 
sides, it has long been plain that the pebbles of these conglomerates are 
simply waterworn fragments of some massive acid rocks, which could never 
have been far removed from where the pebbles now are. It has been sup- 
posed by some that the original massive rocks were to be looked for in the 
older so-called Huronian, although wherever this Huronian is exposed in 
the Lake Superior country such acid rocks are noticeably wanting. 
I find the source of the pebbles in the massive acid rocks of the series 
itself, and recognize now for the first time that these original acid rocks are 
one of its most prominent features, characterizing it, as they do, through- 
out its entire extent, although always subordinated in quantity to the basic 
kinds? I find it even possible to trace some of the pebbles of the con- 
glomerates to their immediate sources. Pumpelly has shown* that the 
-same belt of conglomerate will vary in its predominant pebbles in differ- 
ent portions of its longitudinal extent, while several conglomerates in one 
section will often show the same characteristic pebbles, facts which are to 
be explained by the differences in the original rocks at different points along 
the trend of the formation, and the derivation of the pebbles of the several 
1 Op. cit., p. 99. 
2 Foster and Whitney (Report on the Lake Superior Land District, Vol. I, pp. 65 and 70) speak 
of ‘“‘quartzose porphyry” and ‘‘jasper” as occurring at Mount Houghton, on Keweenaw Point, and in 
the Porcupine Mountains, but they do not seem to have appreciated the true nature of these rocks, which, 
moreover, they regarded as merely alterations of the red sandstones by the heat of the intrusive rocks. 
Macfarlane distinctly recognizes the existence of true quartzose porphyry and of ‘‘trachyte” and 
“phonolite,” on Michipicoten Island (Report of the Geological Survey of Canada for 1866, pp. 137-143), 
but he does not appear to have realized the importance of his observation. 
Hunt has recently spoken of a true quartzose porphyry as occurring on a small island near Saint 
Tgnace Island, on the North Shore, but he appears to regard this as Huronian, though it is undoubtedly 
merely one of the numerous instances of the occurrence of this rock within the Keweenaw Series. 
(Second Geological Survey of Pennsylvania. Special Report on the Trap-Dykes and Azoic Rocks of 
S. E. Penna., § 372, p. 193, and § 446, p. 229.) In my work in the Bad River country of northern Wis- 
consin, in the years from 1873 to 1877, I myself had also recognized true granites cutting gabbro at 
the base of the Keweenaw Series, and also noted and mapped two or more belts of apparently massive 
quartzose porphyry and felsite; but these latter were so poorly exposed—the deceptively massive ap- 
pearance of some of the conglomerates being well known to me—that I only provisionally announced 
the existence of massive acid rocks in my published results (Vol. III, Geology of Wisconsin, pp. 11 and 
193-198. ) 
3Geological Survey of Michigan, Vol. I, Part II, p. 16. 
