98 T. Mellard Reade—G'eological Time. 
of years which it would take to directly produce this sediment must 
be multiplied by the number of times the particles have been used. 
Let us see if we can apply these considerations in estimating a 
section of geological time. 
For the sake of precision we will commence at the base of the 
Cambrian, as the underlying Archean rocks are involved in some 
obscurity, though of late years a very considerable advance has been 
made towards their interpretation. 
A Mops or Estimating THE Time THAT HAS HLAPSED SINCE THE 
BreGInNING OF THE CAMBRIAN. 
Area of Denudation entering into the Calculation. 
Our point of departure being the time when the first Cambrian 
sedimentation began, it will be clear on a little consideration, that 
at the commencement, the area of denudation was necessarily the 
whole of the then land area of the globe, which for our calculation 
we will assume to have been of the same extent as now. But as 
time progressed and fluctuation of levels and mountain building 
began, these Cambrian sediments became land, and to a large extent 
supplied the sediment for succeeding formations. Thus the Silurian 
would be partly formed from the débris of the Cambrian, the Devonian 
from the Cambrian and Silurian, and so on through Carboniferous, 
Permian, Triassic, Jurassic, Cretaceous, Eocene, Miocene, Pliocene, 
and Quaternary. It is thus evident that the pre-Cambrian rocks, 
as they became covered with newer rocks, must have supplied 
sediment to the succeeding formations in gradually decreasing pro- 
portion. What mean proportion these exposures of pre-Cambrian 
rocks throughout the time that has elapsed from the Cambrian 
inclusive to the present, bore to the total land area of the globe 
must be largely a matter of conjecture; but to these exposures, for 
our purposes, must be added all igneous rocks which have come to 
the surface since, and have by their denudation added to the post- 
Cambrian sediments. For the sake of our calculation we will assume 
that the pre-Cambrian and igneous rocks throughout this time were 
as a mean one-third the area of the whole land of the globe. This 
then will constitute the area of denudation, or quarry, so to speak, 
yielding the materials for building up the post-Archzan rocks, the 
other two-thirds being Cambrian and succeeding rocks yielding no 
fresh materials to increase the bulk of post-Archzean sediments. 
Actual bulk of the sediments which have accumulated since the beginning 
of the Cambrian. 
There are several difficulties in estimating the actual bulk of the 
sedimentary rocks of the globe. These difficulties are of two kinds, 
as follows: 
First as to thickness. It is usual in geological works to give the 
thickness compiled from the areas where the maximum development 
of the various formations occurs. Of course every geologist knows 
that at no one place on the earth’s surface do all the formations 
occur together, much less in their greatest development. To put 
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