THE PARALLEL ROADS OF GLEN ROY. 165 
with scientific men. It was considered probable by Poisson 
that our system, during its motion, had traversed portions ' <^, 
of space of different temperatures; and that, during its 4? j, 
passage through one of the colder regions of the universe, 
the glacial epoch occurred. Notions such as these were 
more or less current everywhere not many years ago, and I 
therefore thought it worth while to show how incomplete 
they were. Suppose the temperature of our planet to be 
reduced, by the subsidence of solar heat, the cold of space, 
or any other cause, say one hundred degrees. Four-and- 
twenty hours of such a chill would bring down as snow 
nearly all the moisture of our atmosphere. But this 
would not produce a glacial epoch. Such an epoch would 
require the long-continued generation of the material from 
which the ice of glaciers is derived. Mountain snow, the 
nutriment of glaciers, is derived from aqueous vapor raised 
mainly from the tropical ocean by the sun. The solar fire 
is as necessary a factor in the process as our lamp in the 
experiment referred to a moment ago. Nothing is easier 
than to calculate the exact amount of heat expended by the 
sun in the production of a glacier. It would, as I have 
elsewhere shown,* raise a quantity of cast iron five times 
the weight of the glacier not only to a white heat, but to 
its point of fusion. If, as I have already urged, instead of 
being filled with ice, the valleys of the Alps were filled 
with white-hot metal, of quintuple the mass of the present 
glaciers, it is the heat and not the cold, that would arrest 
our attention and solicit our explanation. The process of 
glacier making is obviously one of distillation, in which 
the fire of the sun, which generates the vapor, plays as 
essential a part as the cold of the mountains which con- 
denses it.f 
It was their ascription to glacier action that first gave 
the parallel roads of Glen Koy an interest in my eyes; and 
in 1867, with a view to self-instruction, I made a solitary 
*"Heata Mode of Motion," fifth edition, chap, vi.: Forms of 
Water, 55 and 56. 
fin Lyell's excellent " Principles of Geology," the remark occurs 
that " several writers have fallen into the strange error of supposing 
that the glacial period must have been one of higher ruean temper- 
ature than usual." The really strange error was the forgetfulness 
of the fact that without the heat the substance necessary to the 
production of glaciers would be wanting. 
