316 Probable Origin and Age of the Sun. [July, 
the amount of rock removed during the Old Red Sandstone 
period was much greater than one mile ; for we know per- 
fectly well that over large tradts of country nearly a mile in 
thickness of rock was carried away between the period of 
the Lower Old Red Sandstone and the Carboniferous epoch. 
Further, all geological fadts go to show that the time repre- 
sented by the Lower Old Red Sandstone itself must have 
been enormous. 
Now, three miles of rock removed since the commencement 
of the Old Red Sandstone period (which in all probability 
s an under-estimate) would give us 45 million years. 
Again, going further back, we find the lapse of time 
represented by the Silurian period to be even more striking 
than that of the Old Red Sandstone. The uncomformities 
in the Silurian series indicate that many thousands of feet 
of these strata were denuded before overlying members of 
the same great formations were deposited. And again, this 
immense formation was formed in the ocean by the slow 
denudation of pre-existing Cambrian continents, just as these 
had been built up out of the ruins of the still prior Lauren- 
tian land. And even here we do not reach the end of the 
series, for the very Laurentians themselves resulted from 
the denudation not of the primary rocks cf the globe, but of 
previously existing sedimentary and probably igneous rocks 
of which, perhaps, no recognisable portion now remains. 
Few familiar with the fadts of geology will consider it too 
much to assume that the time which had elapsed prior to 
the Old Red Sandstone was equal to the time which has 
elapsed since that period. But if we make this assumption, 
this will give us at least go million years as the age of the 
stratified rocks. 
That the foregoing is not an over-estimate of the probable 
amount of rock removed by sub-aerial denudation during past 
geological ages will appear further evident from the following 
considerations : — The mountain ridges of our globe, in most 
cases, as is well known, have been formed by sub-aerial de- 
nudation : they have been carved out of the solid block. 
They stand two thousand, four thousand, or five thousand 
feet high, as the case may be, simply because two thousand, 
four thousand, or five thousand feet of rock have been de- 
nuded from the surrounding country. The mountains are 
high simply because the country has been lowered. But it 
must be observed that the height which the mountains reach 
above the surrounding country does not measure the full 
extent to which the country has been lowered by denudation, 
because the mountains themselves have also been lowered. 
