BY R. M. JOHNSTON, F.LS. 1 
or conglomerate, not one of the whole series containing 
irregular blocks of foreign material, boulders, or gravel, 
such as we have seen to be the essential characteristic 
of a glacial epoch; and when we find that this ‘very 
same general character pervades all the extensive 
tertiary deposits of temperate North America, we shall, I 
think, be forced to the conclusion that no general glacial 
epoch could have occurred during their formation.’” And 
Dr. Wallace further anticipates Sir Robt. Ball’s argument, 
which relies solely upon the ‘“‘imperfection of the geological 
record,” by the concluding part of his observations, where he 
states: “It must be remembered that the ‘imperfection of the 
geological record’ will not help us here, because the series 
of tertiary deposits is unusually complete, and we must 
suppose some destructive agency to have selected all the 
intercalated glacial beds, and to have so completely made 
away with them that not a fragment remains, while preserving 
all, or almost all, the zz¢erg/acial beds; and to have acted thus 
capriciously, not in one limited area only, but over the whole 
Northern Hemisphere, with the local exceptions on the flanks 
of great mountain ranges already referred to.” On the 
whole, therefore, it seems to be conclusively demonstrated that 
a concurrence of favourable geographical conditions with 
astronomical causes is essential to the initiation of glacial 
conditions even in existing temperate zones, and that changes 
of eccentricity, however great, have no potency in themselves 
to produce glaciation of an intense form on the lower levels 
of existing temperate latitudes, because warm air and ocean 
currents have so preponderating an influence, that, if they 
were not diverted and barred by physical and geographical 
conditions from the regions affected, glaciation on lowlands 
of existing temperate latitudes would be impossible. These 
conclusions are in no way disturbed by the more recent 
calculations by Sir Robt. Ball in regard to the exact pro- 
portion of direct heat received by any one hemisphere during 
the long winter and short summer of a period of great 
eccentricity of winter in aphelion in the respective hemispheres; 
for he himself acknowledges that the heat influence affecting 
either hemisphere under the most extreme conditions are not 
confined to the direct sun rays. At page 126 (“The Cause of 
an Ice Age’”’) Sir Robt. Ball states, ‘‘ There are two causes by 
which the severity of a glaciation is somewhat modified. 
There is, first, the actual storage of some of the copious 
heat of summer in the glaciated hemisphere itself, to be doled 
out again during winter ; there is, secondly, the she contribution 
of heat from the opposite hemisphere, which may be conveyed via 
air or via water across the equator into glaciated regions.” Hence 
it follows that his calculations—proving that within the same 
hemisphere the heat received direct from the sun amounts to 
