140 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
stone bed has been removed and large surfaces of the underlying diabase, 
sometimes many hundreds of feet in length, present the singular appearance 
of being intersected by veins of sandstone, the seeming veins crossing each 
other, zigzagging and branching, like true vein-formed material. 
On the Minnesota shore, where single beds can be followed on ex- 
posure for long distances, numerous minor bowings and corrugations are 
seen to affect the layers, which, nevertheless, preserve a constant lakeward 
slant at a low angle. Individual layers and sets of layers can be followed 
for miles, rising into arches, sometimes of short span, and again sinking 
out of sight to reappear in a short distance. These minor undulations are 
the only possible foundation Norwood could have had for his profile of 
the North Shore,’ on which he represents a series of much-folded rocks. 
This peculiar warped structure I conceive to be in a measure due to the 
original irregularities of the beds.” 
Laterally, the beds are, of course, not of indefinite extent. They 
must of necessity be far less extensive than sedimentary beds of the same 
thickness, and these, too, wedge out laterally. With a succession of beds 
much like each other, it is commonly difficult to prove the continuity or 
non-continuity of single flows over any great distance. On the Minnesota 
shore, however, I was able to trace individual layers for ten to fifteen miles 
with certainty, and with great probability much further than this. The 
Greenstone of Keweenaw Point, a melaphyr or olivine-diabase flow of 
considerable thickness, which can be readily traced by reason of its pecu- 
liar character and marked effect on the topography, runs beyond question 
from near the end of Keweenaw Point westward to the Allouez mine, a 
distance of nearly 30 miles, and in all probability is represented at Portage 
Lake, 30 miles further southwest, by a coarse-grained bed seen near the 
Atlantic mine. The so-called Ashbed of Keweenaw Point appears to 
have been recognized at points 30 miles from each other, but the reference 
of the rocks of the two places to the same bed is made on stratigraphical 
and lithological evidence only, no actual continuity having been proved. 
Groups of layers of allied lithological characters are recognizable over 
1Atlas to Owen’s Geological Survey of Wisconsin, Iowa, and Minnesota. 
2See Chapter VII for more specific descriptions of these irregularities, with illustrations. 
