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through it, to the waters and currents of the Northern Sea. oom 
Cretaceous period, however, the Laurentian axis had probably acqu 
its full height from local elevatory action, while its actual summit must — ¥ 
have been much higher than at present, as it had not then suffered the — va 
extensive denudation of Tertiary time, and of the glacial period. Tt i. 
would, therefore, stand sufficiently above the water, with such a degreeof _ q 
continental submergence as that indicated, to form an efficient barrier to — aa eas 
the north-east, and yet not present a sufficient surface of its own to oe 
sub-aerial denudation, to supply, under ordinary circumstances, much de- 
trital matter to the surrounding seas. Even at this time, the region of the 
future Rocky Mountains, must have been marked out by shallows and 
banks, and perhaps also by small areas of dry land, for the calcareous de- | 
posits do not retain the character of their eastern developments, to their 
western borders, but are to a great extent replaced by arenaceous and 
other mechanical sediments ; while still containing a sufficient number of 
the characteristic fossils of the Niobrara division to prove their age. 
379. Between this division of the Cretaceous, and the next in order of 
2 time—that known as the Fort Pierre group to the South, and represent-_ 
ed by the Pembina Mountain group in Manitoba, and also extensively 
: westward, to the north and south of the Line—there exists an evident — 
break, which is not alone marked by a paleontological change, but by the 

; sudden substitution of fine homogeneous argillaceous sediments, with — 
x scarcely a trace of calcareous matter, for those consisting almost entirely | 
i of the calcareous remains of marine life. The surface of the Niobrara 
; deposits, has also been noticed, in some places, to have suffered erosion 
before the deposition of the first beds of the next group. Jam not aware 
that proof exists of the elevation of any part of the area, to such an ex- 
tent as to permit sub-aerial denudation, and it is probable, that the surface 
of these soft calcareous beds, was furrowed by the action of rather strong — 
marine currents; which now, from some change in the physical geography 
of the period, gained access to the region, Whether this change was 
brought about by the opening of an easy communication from the waters 
of the northern to the southern ocean, between the Rocky Mountain Le- 
: gion and the Laurentian land, by the submergence of some barrier till 
that time existing, it is impossible to say with any degree of certainty. 
This would, however, seem to offer the most tenable solution, and it is 
further probable, that this change was caused by a general movement of 
depression which brought rocks decayed by sub-aerial weathering, under 
the action of the sea, and served to supply ready-made material. In any 
