288 Proceedings of the Royal Physical Society. 
gas rise annually from the vicinity of the lake of Laach 
alone, and it is almost certain that quantities equally 
large are given off from many other volcanic areas, both 
active and quiescent, elsewhere. This is mentioned here as 
an answer to one objection which has been often advanced 
against basing an estimate of the Age of the Earth upon the 
rate of denudation now taking place; the ground upon 
which the objection was founded being that the quantity of 
carbon now locked up in our coals and limestones must 
formerly have existed in the atmosphere over and above the 
percentage which is now present. This excess (if present) 
would, as has been very rightly pointed out, most materially 
accelerate at least the chemical disintegration of rocks, and 
thereby indirectly increase also the rate of deposit. But for 
the reasons given here and above, I agree with those who 
think that we are not justified in assuming the existence of 
any such excess above the percentage just given, 22, an 
average of 04 /, ranging to a maximum of ‘10 7%. 
Many coal-seams are known to consist largely of spores 
and spore-cases of Lepidodendroid origin—the actual plants 
themselves constituting but a small percentage of the mass. 
This is not the place to enter into a discussion of the precise 
conditions under which the majority of coal-seams were 
formed,! but on any view of their origin this fact is one of 
the many that points to their extremely slow growth. Mr 
Kidston, in his Address to this Society in 1894, has shown 
us in the clearest possible manner that it is very far from 
being the truth that the same species of plants existed all 
through the Carboniferous Period, and therefore gave rise to 
the various coal-seams that occur from the middle of the 
Lower Carboniferous Rocks up to the highest coal-bearing 
horizon. There was, on the contrary, a gradual replacement 
of one species by another, which, as the physical conditions 
remained the same throughout all the subdivisions of the 
periods when coal-seams were being formed, points in the 
most unmistakable manner to the lapse of enormous intervals 
of time. What the actual time required for the formation 
1 «© Some of the Modes of Formation of Coal-Seams,” J. G. Goodchild, 
Proceedings Royal Physical Society, vol. x., p. 97. 
