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 

 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 Roy an interest in my eyes; and 

 in 1867, with a view to self-instruction, I made a solitary 



* " Heat a 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 mean 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. 



