KEWEENAWAN PERIOD.


is clearly indicated by the present outcrops. If the granite intru-
sions of the Penokee and other regions rose to the surface, they
would then probably take the form of porphyries, instead of gran-
ites, according to the accepted doctrine that igneous granitoid rocks
are formed under pressure, and hence usually at considerable depths,
the same material, under the less pressure at the surface, taking the
form of rhyolite, porphyry, trachyte, and similar imperfectly crys-
talline forms. Porphyries, so formed, might, by their complete
degradation, yield the required amount of detrital material.
The Shates. The shales of the formation are only the finer prod-
uct of the wear of the beds that lay around the margin of the
basin. The character of the shales indicates that, like the sand-
stones and conglomerates, they were quite largely derived from the
igneous rocks of the series. Certain probable circumstances of
eruption may have greatly facilitated their formation. It is clear
that the lava sheets were frequently submerged between periods of
eruption. In instances in which they were thus covered at the time
of the succeeding outflow, the sheet of molten matter spreading out
beneath the water would suffer sudden cooling at the surface from
contact with the water, resulting in the shattering of the external
portion into fragments, so that the surface would be covered by ma-
terial favorably disposed for effective action and further reduction
by the waves. The surface of the lava-floxws seems to indicate that
this was not a uniform circumstance, but may have been a not
uncommon one.
The formation of volcanic ash and scoria in the vicinity of fissures
of eruption, even though not distinctly of the crater type, may have
been an additional source of ready-formed fragmental material.
Composite Action of the Period. The age was, therefore, one in
which igneous and aqueous agencies united with nicely adjusted
conditions of exposure, subsidence, and submergence, to produce a
series of beds, which, in massiveness and rapidity of accumulation,
have rarely been equaled in geological history. At first the fiery
element held sway, and flow after flow welled forth and spread out
over the great area of the basin, while the never-ceasing, but less ob-
trusive, as well as less impulsive, action of aqueous agencies cast in
here and there its contribution of detrital material. But, in time,
the outpourings of lava slackened, the intervals of eruption became
more distant, and their products less massive, while erosion and sedi-
mentation relatively increased, and sandstones, shales, and conglom-
erates became more frequent and thicker. At length, the eruptions
ceased, and over the whole, in Wisconsin, at least, and to some ex-
tent beyond, there was formed the great conglomerate, after which


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