206 
ISLAND LIFE. 
[part t. 
contemplated can be granted. From a consideration of the 
possible sources of the heat of the sun, as well as from calcula- 
tions of the period during which the earth can have been 
cooling to bring about the present rate of increase of tempera- 
ture as we descend beneath .the surface, Sir William Thomson 
concludes that the crust of the earth cannot have been solidi- 
fied much longer than 100 million years (the maximum possible 
being 400 millions), and this conclusion is held by Dr. Croll and 
other men of eminence to be almost indisputable. 1 It will 
therefore be well to consider on what data the calculations of 
geologists have been founded, and how far the views here set 
forth, as to frequent changes of climate throughout all geological 
time, may affect the rate of biological change. 
Denudation and Deposition of Strata as a measure of Time . — The 
materials of all the stratified rocks of the globe have been ob- 
tained from the dry land. Every point of the surface is exposed 
to the destructive influences of sun and wind, frost, snow, and 
rain, which break up and whar away the hardest rocks as well 
as the softer deposits, and by means of rivers convey the worn 
material to the sea. The existence of a considerable depth of soil 
over the greater part of the earth’s surface ; of vast heaps of 
rocky debris at the foot of every inland cliff ; of enormous 
deposits of gravel, sand, and loam ; as well as the shingle, 
pebbles, sand, or mud, of every sea-shore, alike attest the uni- 
versality of this destructive agency. It is no less clearly shown 
by the way in which almost every drop of running water — 
whether in gutter, brooklet, stream, or large river — becomes 
discoloured after each heavy rainfall, since the matter which 
causes this discolouration must be derived from the surface of 
the country, must always pass from a higher to a lower level, 
and must ultimately reach the sea, unless it is first deposited in 
some lake, or by the overflowing of a river goes to form an 
alluvial plain. The universality of this subaerial denudation, 
both as regards space and time, renders it certain that its cumu- 
lative effects must be very great ; but no attempt seems to have 
1 Trans. Royal Society of Edinburgh, Vol. XXIII. p. 161. Quarterly Jour- 
nal of Science, 1877. (Croll on the “ Probable Origin and Age of the 
Sun.”) 
