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This seems next to have been followed in the interior continental x 
by a gradual subsidence, during which the second division was foe 
The cessation of subsidence—or possibly even a slight movement of e- 
vation—and the advent of a prolonged period of comparative tranquility, Ys 
with certain other favouring circumstances, gave rise to the Niobrare a 
limestone formation. The indications next favour the belief of a secon ad 
subsidence, admitting marine currents and bringing onimportant chan ges 
in physical geography. During this period, according to Meek and Hay-— : 
den, a part of the clays and green-sands of New Jersey, were formed on os 
the Atlantic coast; and in the Mississippi region, according to Hilgard, — : 
the Rotten-limestone group was deposited. The fact that calcareous beds 
were being formed in the southern portion of the interior continental 
trough, while such purely argillaceous deposits were produced in the north- 
ern; would also tend to verify the supposition of the entrance of — ‘ 
sediment-bearing waters from the north. Closing the Cretaceous, pro- — 
perly so-called, the fifth division shows the resumption of shoal water and 
sand-bank over the entire northern portion of the interior, a change pro-— 
bably at least aided by elevation. But that the decreased depth was per- 
haps mainly due to the filling up of the northern portion of the basin, - 
seems to be indicated by the fact, that limestones and marlites still con- 
tinued to be deposited in the Mississippi region. 
Ho 
382. The early Cretaceous period, from the evidence chiefly of the 
plants of the Dakota group, must have had a temperate climate. During 
the succeeding ages, the heat was no doubt becoming greater, for though 
there is no complete succession of land plants, we find a well-marked max- 
imum of temperature, as stated by Prot. Lesqereux, in the early period 
of the Lignite Tertiary, and from that time the change was back again to 
a climate resembling that of the Dakota group, if not even cooler. 
383. Prof. N. H. Winchell, in his last published report, announces 
the discovery of extensive beds of gvanite decomposed in place, and 
kaolin-like material, covering the oldest rocks of south-western Minne- 
sota, wherever they have been protected by overlapping Cretaceous beds from 
the scouring of the drift period.* This throws much light on the origin of 
the fine clayey, and coarser sandy deposits of the Cretaceous, and shows 
that the rocks of the Laurentian axis must have been much decomposed 
by sub-aerial action prior to the opening of the Cretaceous period, and 
that these soft materials brought under the action of the waves during 

* Second Annual Report, Geol. Surv. Minn. 
