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they indicate a capability of yielding to strain such as might be 
supposed hardly possible in a globe possessing on the whole the rigidity 
of steel or glass. Still we ought to remember how small a part of 
the whole terrestrial area is occupied by those portions of land 
from the investigation of which all our direct evidence as to the — 
nature of the earth’s crust has been obtained. From the earliest 
times the existing continental regions seem to have specially suffered 
from the efforts of the planet to adjust its external form to its — 
diminishing diameter, and its lessening rapidity of rotation. They — 
have served as lines of relief from the strain of compression 
during many successive epochs. It is along their axial lines,—their 
long dominant mountain ranges, that we should naturally look for 
evidence of corrugation. Away from these lines of weakness the — 
ground has been upraised for thousands of square miles without 
plication of the rocks, as in the instructive region of the Western — 
Territories of North America. Nor is there any sign that corrugation 
takes place beneath the great oceanic areas of subsidence. | 
It appears highly probable that the substance of the earth’s 
interior is at the melting point proper for the pressure at each depth. 
Any relief from pressure therefore may allow of the liquefaction of the ~ 
matter so relieved. Such relief is doubtless afforded by the corruga- 
tion of mountain chains and other terrestrial ridges, And it is in 
these lines of uprise that volcanoes and other manifestations of sub- 
terranean heat actually show themselves. 
§ 4. Age of the Earth and Measures of Geological Time.— 
The age of our planet is a problem which may be attacked either 
from the geological or physical side. a 
1, The geological argument rests chiefly upon the observed 
rates at which geological changes are being effected at the present 
time, and is open to the obvious preliminary objection that it assumes 
the existing rate of change as the measure of past revolutions,—an _ 
assumption which may be entirely erroneous, for the present may be 
a period when all geological events march forward more slowly than 
they used to do, The argument proceeds on data partly ofa physical 
and partly of an organic kind. (a.) The physical evidence is deriyed 
from such facts as the observed rates at which the surface of a coun- 
try is lowered by rain and streams, and new sedimentary deposits 
are formed. ‘These facts will be more particularly dwelt upon in 
later sections of this volume. If we assume that the land has been 
worn away, and that stratified deposits have been laid down nearly 
at the same rate as at present, then we must admit that the strati- 
fied portion of the crust of the earth must represent a very vast 
period of time’ (b.) On the other hand, human experience, so 
' Dr. Croll puts this perio 18 ssi Aue , See 
year Br Hiden taro not Tes, bok pony much more Ub 60 sling 
of deposit of strata at L foot in 8616 years, assuming the former rate to have been ten’ 
times more rapid, or 1 foot in 861°6 years, and taking the thickness of the stratified rocks 
of the earth’s crust at 177,200 feet, he obtains a minimum of 200,000,000 
! : ; Be mk years f 
whole duration of geological time: Six Lectures on Physical Geography, 1880, p. 94. i 
