6 
W. J. Sollas — On Evolution in Geology. 
stantly diminishes as we proceed from the more recent to the older 
rocks ; and certainly the lengtli of time required for the formation 
of the Palaeozoic rocks, calculated according to their thickness and 
the rate of existing processes of deposition, would be far in excess 
of that derived from a consideration of the changes in their fossil 
fauna. That this might indicate a quicker rate of deposition I 
pointed out in my lectures two years ago, while a previous writer 
has endeavoured to prove the converse proposition, viz. that the 
changed proportion indicates that the rate of Variation amongst 
animals and plants must have progressively increased from the 
earliest to the latest times. Certainly the continual increase of 
the sum total of species, an increase only partially checked by the 
extinction of some species, will give greater opportunities for 
Variation ; but beyond this there appears but little to explain a 
progressively increasing rate of Variation which should affect all the 
great classes of animals alike ; and since the organic world seems 
to change much less quickly than the mineral one, one feels much 
more inclined to attribute the greater thickness of the older rocks, in 
proportion to the number of their contained species, to an accele- 
rated rate of deposition, than the contrary. If this be the correct 
view, and the change in proportion be available as a means of 
measurement, we sliall then find that the Palfeozoic rocks were 
deposited three times as rapidly as the Cainozoic strata, since they 
contain one quarter the number of species of fossils for the same 
given thickness of rocks . 1 
Metamorphosis. — This results from the descent of Sediments towards 
the interior of the earth, by which they become exposed to a tempera- 
ture high enough, under the circumstances, to alter their character. 
The readier metamorphosis of the earlier rocks follows naturally 
from the more rapid rise of temperature which then occurred for a 
given descent into the earth’s crust from its surface, and from the 
greater activity of crust-movements, which we shall show charac- 
terized the earlier epochs. 
Using Sir William Thomson’s estimates for au illustration of the 
former influences, we find that 4,000,000 years after permanently 
incrusting, the temperature-increase for every foot descended into 
the crust is : and tlius a descent of rocks 100,000 ft. from the 
surface — by no means an improbable amount — would bring them into 
regions the temperature of which would at least amount to 10,000° F., 
by which of course they would necessarily be fused and re-absorbed 
into the melted interior of the earth. 
1 The studious reader who desires to enter more fully into this question of the 
“ Rate of Geologieal Change ” should consult Prof. Huxley’s Presidential Address 
to the Geologieal Society (18691, Quart. Journ. Geol. Soc., yoI. xxv. ; Prof. E. 
Forbes “ On the Manifestation of Polarity in the Distribution of Organized Beings 
in Time ” (Royal Inst. Lectures, vol. i. p. 428) ; Prof. Phillips’ Rede Lecture : 
Cambridge, 1860, afterwards published hy Macmillan under the title “ Life on the 
Earth,” 1860 ; and an able article by Mr. H. M. Jenkins, F.G.S., Sec. Royal Agl. 
Soc. of England, “ On the Rate of Geologieal Change,” Quart. Journ. Science, 
1870, vol. vii. p. 322. — Edit. Geol. Mag. 
