TRANSACTIONS OF SECTION D. 815 
of the two other lines of reasoning. Indeed, Professor George Darwin, although 
not feeling the force of these latter, agrees with Lord Kelvin in regarding 500 
million years as the maximum life of the sun.! 
We may observe, too, that 500 million years is by no means to be despised; a 
great deal may happen in such a period of time. Although I should be very sorry 
to say that it is sufficient, it is a very different offer from Professor Tait’s 
ten million. 
In drawing up this account of the physical arguments, I owe almost everything 
to Professor Perry for his articles in ‘ Nature’ (January 3 and April 18, 1895), 
and his kindness in explaining any difficulties that arose. I have thought it right 
to enter into these arguments in some detail, and to consume a considerable pro- 
portion of our time in their discussion. This was imperatively necessary, because 
they claimed to stand as barriers across our path, and, so long as they were admitted 
to be impassable, any further progress was out of the question. What I hope has 
been an unbiassed examination has shown that, as barriers, they are more imposing 
than effective; and we are free to proceed, and to look for the conclusions warranted 
by our own evidence. In this matter we are at one with the geologists; for, as 
has been already pointed out, we rely on them for an estimate of the time occupied. 
by the deposition of the stratified rocks, while they rely on us for a conclusion as to 
how far this period is sufficient for the whole of organic evolution. 
First, then, we must briefly consider the geological argument, and I cannot do 
better than take the case as put by Sir Archibald Geikie in his Presidential Address 
to this Association at Edinburgh in 1892. 
Arguing from the amount of material removed from the land by denuding 
agencies, and carried down to the sea by rivers, he showed that the time required 
to reduce the height of the land by one foot varies, according to the activity of the 
agencies at work, from 730 years to 6800 years. But this also supplies a measure 
of the rate of deposition of rock; for the same material is laid down elsewhere, 
and would of course add the same height of one foot to some other area equal in 
size to that from which it was removed. 
The next datum to be obtained is the total thickness of the stratified rocks 
from the Cambrian system to the present day. ‘On a reasonable computation 
these stratified masses, where most fully developed, attain a united thickness of 
not less than 100,000 feet. If they were all laid down at the most rapid recorded 
rate of denudation, they would require a period of seventy-three millions of years 
for their completion. If they were laid down at the slowest rate, they would 
demand a period of not less than 680 millions,’ 
The argument that geological agencies acted much more vigorously in past 
times he entirely refuted by pointing to the character of the deposits of which the 
stratified series is composed. ‘We can see no proof whatever, nor even any 
evidence which suggests that on the whole the rate of waste and sedimentation 
was more rapid during Mesozoic and Paleozoic time than it is to-day. Had there 
been any marked difference in this rate from ancient to modern times, it would be 
— that no clear proof of it should have been recorded in the crust of the 
earth. 
It may therefore be inferred that the rate of deposition was no nearer the more 
rapid than the slower of the rates recorded above, and, if so, the stratified rocks 
would have been laid down in about 400 million years. 
There are other arguments favouring the uniformity of conditions throughout 
the time during which the stratified rocks were laid down, in addition to those 
which are purely geological and depend upon the character of the rocks themselves. 
ee etae more biological than geological, these arguments are best) considered 
ere. 
The geological agency to which attention is chiefly directed by those who desire 
to hurry up the phenomena of rock formation is that of the tides, But it seems 
* British Association Reports, 1886, pp. 514-518. 
