310 The History of our Earth. [July, 
undergone important secular alterations. Fora considerable 
time it was supposed that during the Cambrian, Silurian, 
and other pristine periods, the climate of our globe was 
much hotter than at present, and that it has ever since been 
gradually declining, as the internal regions of the earth have 
become progressively colder. Modern physical research, 
however,* has shown that the earth’s general climate could 
not have been appreciably modified by internal heat for more 
than 10,000 years after its surface began to solidify, and 
that the present influence of internal heat upon the tem- 
perature does not exceed the 1-75th of a degree. Further, 
geological evidence goes to prove that—so far from a unl- 
form or progressive decline of temperature having taken 
place from the earliest ages—there have been periods when 
the climate was very much colder than at present. There 
have been, in fact, not one, but several ‘‘ Glacial periods,”— 
times when an aréctic condition of climate prevailed in the 
British islands, and when they, along with the greater part 
of the European continent, were buried under ice. Between 
these periods, again, there intervened others, when Western 
Europe enjoyed a semi-tropical climate, whilst even Green- 
land and Nova Zembla were free from ice, and covered with 
luxuriant vegetation. 
It must be remembered that a Glacial epoch is not merely 
a season of unusual cold. Its characteristic feature is the 
‘“‘ slaciation”’ of a considerable portion of the land,—in 
other words, its becoming coated with a continuous mass of 
glacier-ice. This phenomenon cannot, obviously, arise from 
any general decrease of the earth’s temperature. Were the 
sun to be extinguished, evaporation from the surface of the 
globe would cease, and all downfall, either of rain or 
snow, would cease likewise. Though, therefore, in such an 
assumed case the ocean would be frozen down to its very 
bed, the land would remain bare of ice. For glaciation it is 
necessary that evaporation should go on unhindered in some 
warm parts of the globe, and that the moisture thus volati- 
lised should be transferred to colder regions, and there pre- 
cipitated. This consideration at once disposes of a variety 
of theories concerning the probable cause of a Glacial epoch, 
as we shall see hereafter, and proves—what no strictly geo- 
logical evidence could show—that no such epoch could 
simultaneously affect both hemispheres. 
This premised, we may proceed to examine some of the 
more important of the numerous hypotheses put forward to 
account for these alternating epochs of heat and cold. 
* Sir WiLLIAM THomson, Phil. Mag., January, 1863. 
