nomena exhibited in the structure of
the earth the student takes up the study of the accumulations of
strata, and endeavours to determine the time required for the laying
down of the sediments, he finds similar evidence of the earth's great
antiquity. Although the process of deposition, which has given us the
rocks visible in the land masses, has been very much interrupted, the
section which is made by grouping the observations made in various
fields shows that something like a maximum thickness of a hundred and
fifty thousand feet of beds has been accumulated in that part of
geologic time during which strata were being laid down in the fields
that are subjected to our study. Although in these rocks there are
many sets of beds which were rapidly formed, the greater part of them
have been accumulated with exceeding slowness. Many fine shales, such
as those which plentifully occur in the Devonian beds of this country,
must have required a thousand years or more for the deposition of the
materials that now occupy an inch in depth. In those sections a single
foot of the rock may well represent a period of ten thousand years. In
many of the limestones the rate of accumulation could hardly have been
more speedy. The reckoning has to be rough, but the impression which
such studies make upon the mind of the unprejudiced observer is to the
effect that the thirty miles or so of sedimentary deposits could not
have been formed in less than a hundred million years. In this
reckoning it should be noted that no account is taken of those great
intervals of unrecorded time, such as elapsed between the close of the
Laurentian and the beginning of the Cambrian periods.

There is a third way in which we may seek an interpretation of
duration from the rocks. In each successive stage of the earth's
history, in different measure in the various ages, mountains were
formed which in time, during their exposure to the conditions of the
land, were worn down to their roots and covered by deposits
accumulated during the succeeding ages. A score or more of these
successively constructed series of elevations may readily be observed.
Of old, it was believed that mountain ranges were suddenly formed, but
there is, however, ample evidence to prove that these disturbed
portions of the strata were very gradually dislocated, the rate of the
mountainous growth having been, in general, no greater in the past
than it is at the present day, when, as we know ful