1 877 .] Probable Origin and Age of the Sun. 317 
The height of the mountains represents merely the extent to 
which the country has been lowered. In the formation of 
a mountain by denudation, say 3000 feet in height, probably 
more than 6000 feet of strata may have been removed from 
the surrounding country. The very faCt of a mountain 
standing above the surrounding country exposes it the more 
to denudation, and it is certainly not an exaggerated assump- 
tion to suppose that whilst the general surface of the country 
was being lowered 6000 feet by denudation, the mountain 
itself was at least lowered by 3000 feet. 
The very common existence of mountains two or three 
thousand feet in height, formed by sub-aerial denudation, 
proves that at least one mile must have been worn off the 
general surface of the country. It does not, of course, fol- 
low that the general surface ever stood at an elevation of 
one mile above the sea-level, since denudation would take 
place as the land gradually rose. We know that the land 
was once under the sea, for it was there that it was formed. 
It is built up out of the materials resulting from the carving 
out into hill and dale, through countless ages, of a previously 
existing land, just as this latter had resulted from the 
destruction of a still older land, and so on in like manner 
back into the unknown past. 
It has now been proved, by the foregoing very simple and 
obvious method, that the age of the earth must be far more 
than 20 or 30 million years. This method, it is true, does 
not enable us to determine with anything like accuracy the 
aCtual age of the globe, but it enables us to determine with 
absolute certainty that it must be far greater than 20 million 
years. We have not sufficient data to determine how many 
years have elapsed since life began on the globe, for we do 
not know the total amount of rock removed by denudation ; 
but we have data perfectly sufficient to show that it began 
far more than twice 20 million years ago. 
But if the present order of things has been existing for 
more than 20 million years, then the sun must have been 
illuminating our globe for that period, and, if so, then there 
must have been some other source than that of gravitation 
from which the sun derived its energy, for gravitation, as we 
have seen, could only have supplied the present rate of 
radiation for about one-half that period. 
It is perfectly true, as has been stated, that the length of 
time that the sun could, by its radiation, have kept the earth 
in a state fit for animal and vegetable life, must have been 
limited by the store of energy in the form of hea 'which it 
possessed. But it does not follow as a necessary conse- 
