22 COSMICAL ASPECTS OF GEOLOGY. [Boor I. 
are. There appears to have been a gradual lowering of the general ; 
temperature during past geological time, accompanied by a tendency i 
towards greater extremes of climate. But there are proofs also — 
that at longer or shorter intervals cold cycles have intervened. The 
glacial period, for example, preceded our own time, and in suc- 
cessive geological formations indications of more or less value have 
been found that point to a prevalence of ice in what are now 
temperate regions. 
Various theories have been proposed in explanation of such 
alternations of climate. Some of these have appealed to a change in © 
the position of the earth’s axis relatively to the mass of the planet — 
(ante, § 5). Others have been based on the notion that the earth 
may have passed through hot and cold regions of space. Others, — 
again, have called in the effects of terrestrial changes, such as the — 
distribution of land and sea, on the assumption that elevation of 
land about the poles must cool the temperature of the globe, while 
elevation round the equator would raise itt But the changes of 
temperature have affected vast areas of the earth’s surface, while — 
there is not only no proof of any such enormous vicissitudes 
in physical geography as would be required, but good grounds for 
believing that the present terrestrial and oceanic areas have remained — 
on the whole on the same sites from very early geological time. — 
Moreover, as evidence has accumulated in favour of periodic alterna- — 
tions of climate, the conviction has been strengthened that no mere — 
local changes could have sufficed, but that secular variations in 
climate must be assigned to some general and probably recurring 
cause. , 
By degrees geologists accustomed themselves to the belief that — 
the cold of the Glacial Period was not due to mere terrestrial 
changes, but was to be explained somehow as the result of cosmical 
causes. Of various suggestions as to the probable nature and — 
operation of these causes, one desetxes careful consideration—change 
in the eccentricity of the earth’$‘azbit. Sir John Herschel? pointed — 
out many years ago that the-dirett effect of a high condition of 
eccentricity is to produce an unusually cold winter followed by a 
correspondingly hot summer on the hemisphere whose winter occurs 
in aphelion, while an equable condition of climate will at the same 
time prevail on the opposite hemisphere. But both hemispheres 
must receive precisely the same amount of solar heat, because the 
deficiency of heat resulting from the sun’s greater distance during — 
one part of the year is exactly compensated by the greater length of 
that season. Sir John Herschel even considered that the direct 
effects of eccentricity must thus be nearly neutralised.2 As a like 
verdict was afterwards given by Arago, Humboldt, and others, 
' In Lyell’s Principles of Geology, this doctrine of the influence of geographical 
changes is maintained. 
* Trans. Geol. Soc., vol. iii. p. 293 (2nd. series). 
* Cabinet Cyclopadia, sec. 315; Outlines of Astronomy, sec. 368. 


