“2 
Cretaceous Peneplain. 69 
level surface and still remain as isolated peaks, ridges or moun- 
tain groups above the remnants of that plain. The distribution 
and relations of these remnants will be more easily understood 
after the Cretaceous peneplain has been described in detail ; 
-hence their consideration will be deferred and included under 
the physiography of the Cretaceous peneplain. 
DEFORMED CRETACEOUS PENEPLAIN. 
The oldest topographic feature that can be identified with cer- 
tainty in this region, one which forms the basis upon which all 
later history has been recorded, is a more or less perfectly pre- 
served baselevel peneplain. The reasons for ascribing its forma- 
tion to Cretaceous time are given in a subsequent part of this 
paper and its Cretaceous age may be assumed for the present. 
Doubtless, at earlier periods the surface of the province had 
been baseleveled again and again, but subsequent erosion has so 
modified these earlier forms as to leave them unrecognizable. 
Conditions of Development.—The condition under which a plain 
of erosion will be formed is long-continued stability of baselevel, 
and as baselevel is usually determined by sealevel, the essential 
condition is that the relative position of land and sea shall re- 
main unchanged for a period long enough to allow the agents of 
erosion to carry their work toward completion and reduce the 
surface of the land to drainage-level, the baselevel of erosion. 
During Cretaceous time the condition of stability prevailed 
in this region for the longest period of which we have any 
record in its history; for, while it is a popular belief that the 
normal condition of the earth’s crust is one of stability, the re- 
verse is. shown to be true of this region. Its history in post- 
‘Paleozoic time is a record of almost continuous orogenic move- 
ment—extremely slow, it is true, but with sufficient time allowed, 
capable of producing the greatest deformations with which we 
are acquainted. 
Throughout this period of exceptional quiet, erosion was in 
progress, reducing the surface toward baselevel—rapidly at first, 
as the land was high and the slopes steep, but at a rate growing 
gradually less and less as the gradient of the streams decreased 
and with it their ability to carry off the waste of the land. As 
the gradient approached its lowest limit the mineral matter 
removed from the land was almost wholly in solution. This 
process continued, reducing to baselevel first the soft and soluble 
