﻿6 W. J. Sollas — On Evolution in Geology. 



stantly diminislies as we proceed from the more recent to the older 

 rocks ; and certainly the length 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 shall then find that the Palaeozoic rocks were 

 deposited three times as rapidly as the Oainozoic strata, since they 

 contain one quarter the number of species of fossils for the same 

 given thickness of rocks.^ 



Metamor pilosis. — 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 an 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 ^V" '• and thus 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 

 " Eate of Geological Change " should consult Prof. Hiixley's Presidential Address 

 to the Geological Society (1869), Quart. Journ. Geol. Soc, vol. xxv. ; Prof. E. 

 Forbes " On the Manifestation of Polarity in the Distribution of Organized Beings 

 in Time" (Eoyal Inst. Lectures, vol. i. p. 428); Prof. Phillips' Eede Lecture: 

 Cambridge, 1860, afterwards published by Macmillan under the title " Life on the 

 Earth," 1860 ; and an able article by Mr. H. M. Jenkins, F.G.S., Sec. Eoyal Agl. 

 Soc. of England, " On the Eate of Geological Change," Quart. Journ. Science, 

 1870, vol. vii. p. 322.— Edit. Geol. Mag. 



