BY R. M. JOHNSTON, F.L.S. 111 
Tae OBJECTIONS WHICH SuaccEest THEMSELVES WHEN 
ATTEMPTING TO EXPLAIN THE PHENOMENA OF GLACIA- 
TION IN BOTH H&rMISPHERES BY REFERENCE TO THE 
AstRONOMICAL THEORY TAKEN BY ITSELF. 
As my object is to go straight to the root of the difficulty 
of explaining the intense form of glaciation over Northern 
Europe and North America during the pleistocene period by 
reference to the astronomical theory alone, I will at once 
grant that the necessary amount of precipitation and vapour, 
the thermal currents of air and ocean, the elevation of the 
land, the distance of inland regions from vapour-procucing 
sources, and the difference in latitude, would result in pro- 
ducing all those variable effects upon isotherms and amount 
of snowfall within the same latitude as they are known to do 
at present; although we must anticipate greater intensity of 
effect, and a greater extension of the isotherm of frost at 
sea level, together with a general lowering of the snow level 
or névé. 
Thus, though it must still be contended that geographical 
and physical causes would continue to operate in producing 
some such variations in isotherms, and in the variableness 
of the distribution of snow, even in the same latitude, or 
even in different faces of great physical barriers such as lofty 
mountain chains, it is still quite conceivable that the astron- 
omical cause alone might adequately account for the glacial 
epoch of Europe and North America, as well as for the milder 
effects of glaciation exhibited in the rocks of Australia. 
The real difficulty to the acceptance of the adequacy of the 
astronomical theory, zaken by itself, is as much due to its lack 
of consistency with astronomical facts as it is to its inadequacy 
to explain the facts of geology, and in the latter case —not 
So much as regards positive evidence in respect of glacial 
effects actually preserved in our rocks, as for its inadequacy to 
explain the absence of glacial effects at numerous points 
within the tertiary period corresponding with the cycles 
of extreme eccentricity* of the earth’s orbit combined 
with winter in aphelion; many of which were as_ great, 
and, at two cycles, enormously greater than in the pleis- 
tocene period, where alone correspondence appears to 
occur. ‘To be consistent with itself, therefore, it must demand 
‘that similar effects of glaciation in both hemispheres in the 
same latitude, or nearly so, should be produced at intervals, 
and in intensity corresponding with the recurrence of 
* This ison the assumption that Dr. Croll’s calculations of periods of eccén- 
tricity calculated for the last 3 millions are relatively correct, if not absolutely so; 
and that there-were many cycles within the cainozoic age alone, two of which’ were’ 
enormously greater than the last; which is-supposed to correspond with the glacial 
epoch of pleistocene age, 
