DIKES. 143 
from the lake shore somewhat more gradually than the dip, to a height of 
over 1,000 feet, and then drops off in a bold escarpment of 400 feet into 
the valley of Carp Lake. Further west again, as far as Bad River, the 
dips are high, often reaching 90°, and the harder rocks constitute merely 
rounded ridges and knobs with the cliffs facing indifferently in all direc- 
tions. Beyond Bad River, and all across Wisconsin to the Saint Croix, the 
dips flatten once more, and the ‘‘saw-tooth” shape in the ridges is every- 
where well marked. 
True cutting masses, or dikes, of the basic rocks occur, though never 
a prominent feature of the series. They appear to characterize only its 
lower portions, and have been seen chiefly along the Minnesota shore, and 
on the east coast in the region of Mamainse. On the South Shore they are 
nearly unknown, though in all probability occurring largely in the lower 
portions, where the exposures are commonly not good. ‘Throughout the 
entire basin they have never been observed more than one-third the way 
above the base of the series. 
As seen on the Minnesota coast! these dikes are always small, com- 
monly under ten feet in width, though seen occasionally wider than this, 
while a number were noticed under two feet in width. They are usually 
provided with a well-marked cross-columnar structure and coarser-grained 
middle portion as compared with the sides. They consist, so far as yet ex- 
amined microscopically, wholly of augite-plagioclase rocks, and are not in 
any essential point different from the bedded diabases and melaphyrs that 
constitute the bulk of the series, the differences being only those that would 
arise from the different situations in which the rocks have cooled, and the 
different decomposition effects. Indeed, the thin sections of the dike rocks 
are often indistinguishable from those of the bedded rocks. The dikes of 
the Minnesota coast trend usually with the strike of the formation, less 
commonly directly across it. I cannot doubt that these dikes and others 
like them all around the rim of the Lake Superior basin, now unseen only 
because of unfavorable conditions of exposure, have been the source of the 
1Jt should be said here that Norwood’s descriptions of the Minnesota coast are entirely mislead- 
ing as to the number and size of the dikes. As already said, small dikes occur somewhat abundantly 
at low horizons, but the greater number of Norwood’s dikes are merely erosion points of the harder and 
more compact portions of the diabase flows. 
