270 Buell—Geology of the Waterloo Quartzite Area. 
Black Hills,* and they are there considered as due to the open¬ 
ing of the grains along these lines by the crushing force and 
the attendant redeposition of quartz imprisoning within them 
the inclusions. 
The Metamorphic Conglomerate .—Some light is thrown upon 
the origin of the formation by an examination of the pebbles 
from the conglomeratic layers. These are chiefly of very pure 
glassy quartz from white to reddish purple or nearly black, but 
among these are gray micaceous and black magnetite frag¬ 
ments. Sections from the latter show that quartz is still the 
principal mineral, with large amounts of mica and iron oxides 
in the interstitial material. Hematite occurs with the mao 1 - 
netite in these sections, the former being distinguished by its 
blood red translucency. The matrix inwrapping these pebbles 
contains a relatively large amount of sericite and gives a more 
decided schistose aspect to the conglomerate than is observed 
in the more homogeneous layers. The presence of these con¬ 
glomerates on the northwest margin of the area may indicate 
the near approach of the strata to the contact zone bordering 
the older formation. 
COMPARISON WITH LAKE SUPERIOR AND OTHER QUARTZITES. 
The evidence of dynamic action accompanying the meta¬ 
morphism of these rocks is strikingly at variance with the more 
common structure of the pre-Cambrian quartzites of the region 
of the Great Lakes and upper Mississippi as described by Dr. 
Van Hise.f He says: 
“About one hundred localities, the most of them of pre-Cam¬ 
brian age, are mentioned in bulletin No. 8 of the U. S. 
Geological Survey, in which the induration of quartzites was 
produced by a process of enlargement of old quartz 
particles or else by the deposition of new quartz between the 
grains rather than a destruction of the original fragments. So 
far as our experience has extended, practically all quartzites 
properly so-called of whatever age, thus reveal their fragmental 
* Bulletin of the Geological Society of America, Vol. I, p. 215, et seq. 
tlbid., p. 213. 
