350 Edward A. Martin— How Long Ago” in Geology. 
Now, dates in geological history cannot be given with any degree 
‘of certainty. The amount of conjecture in proportion to the amount 
of fact must be very large indeed; yet even a rough estimate is not 
without its value, since it may give an idea of the really comparatively 
short space of time in geological history during which some of the 
best-known formations were laid down. 
Geologists will for a long time differ amongst themselves as to 
what term of years has elapsed since a solid crust had formed on the 
surface of our globe, when life as we know it had first begun to be, 
and when denudation and all that it means had commenced to operate, 
and in embossing upon the surface of the earth the scenery as we now 
see it. Things have changed since geologists were asked to limit 
themselves to 20 millions of years as the period during which all 
the sedimentary formations were laid down. Professor Huxley’s 
willingness to confine himself to 100 millions of years is not altogether 
concurred in, now that the discovery of the great energy lying 
dormant in the various radio-active bodies has shown the possibility 
of a much greater age being assigned to the sun as a heating luminary 
over and above that hitherto supposed. I am justified from many points 
of view in assuming that a solid crust had formed about 250 million 
years ago, that strata have formed at an average rate of one foot in 
700 years, and that the older the rocks the more the strata have been 
compressed into thinner layers. We must also not only consider in 
our discussion the number of years which known strata took to form, 
but the years which are represented by the great gaps between the 
formations when no deposition was taking place but when great 
changes were taking place in the evolution of life. 
Adopting Sollas’ measurements of known strata, and placing against 
each formation the number of years which it approximately represents, 
we shall have the following table (see p. 351). For all strata below and 
including the Devonian I have allowed an addition of 50 per cent. for 
compression subsequent to deposition. For the Carboniferous, Permian, 
and Triassic I have added 25 per cent. for the same reason, and from 
the Jurassic to the Oligocene 10 per cent. For deposits between the 
Miocene to the Recent: inclusive I have left the thicknesses as 
estimated by Sollas, since the rate of deposition may in some portions 
have been less than the estimated rate already mentioned, and in other 
parts there may have been compression as in older formations. The 
relative proportion of each formation to the rest will be found in 
column three. 
We shall find from this table that approximately the great Coal age 
came to a close 70,495,000 years ago, and the Chalk age (Cretaceous) 
31,680,000 years ago. It will also be seen that an enormous period 
had elapsed before the earliest formation commenced to be laid down 
in which fossil life is decipherable, viz. Cambrian. ‘The earlier great 
American formations, included in which is the older-named Laurentian, 
show a space of time of 86,100,000 years. 
It is interesting to note the years which have been necessary 
according to this table to evolve the various forms of life. The 
evolution of the trilobites of the Cambrian was accomplished in about 
86 millions of years. The six-feet-long Crustaceans and mudfish of 
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