a igo enh 
Mineralogy and Geology. 115 
2. On the Nature and Cause ie the Glacial Climate; i ; OSEPH 
Joun Murpuy, Esq., F.G.S. (Q. J. Geol. Soc., xxv, 350, 1 869).— 
In the present paper I purpose to show how far I agree with, and 
where I differ from, Mr. Croll as to the views on the cause of the 
glacial climate se forth in his paper in the ‘ Philosophical Maga- 
zine’ for August, 1864, 
Mr. Croll’s conclusions may be stated in the three following 
propositions :— 
glacial period occurs when the eccentricity of the earth’s 
2 orbit i is at a maximum, and the solstices fall when the earth is in 
perihelio and in aphelio, 
Only one hemisphere, the northern or the southern, has a 
glacial climate at the sa 
r on glaciated heailieglatngd is that of which the winter occurs 
in aphe 
I agree with Mr. Croll as to the first two propositions, but differ 
as to aha. third. I believe that the glaciated hemisphere i is that of 
which the swmmer occurs in n aphelio 
The followin propositions are self-evident when sta 
Summer in aphelio, the nearness of the sun in winter will cause a 
— ——e er, and his remoteness in summer will cause a cool 
\ Bbpose, for ror ale th at Ses. the eccentricity of the earth’s 
orbit is much greater than at present, the midwinter of the Northern 
emisphere occurs in perihelio; then 
the Northern hemisphere will have a mild winter and cool summer, 
the Southern hemisphere will have a sbi winter and hot summer. 
So far (granting Mr. Croll’s astronomical data, for which he cites 
tae and which I believe are indimpatalli there is no room 
for dou I have now to diseuss the question, = effect these 
diversities of climate will have in produci ihe glaciatio 
“page — the ree isa of co winter will be the = 
will be the faces one. 
On a subject it is needless to attempt to make any deduction 
cory. We have plenty of observed data; and I think I can 
show that they all go to prove a cool summer to to be what most pro- 
rR Eieciataon, while a cold winter has, usually, no effect on it 
atever, 
ordes, in his work on Norway and its Glaciers, p. 206, quotes 
~ the excellent goneraticsbion of von Buch, that it is the tempera- 
‘ure of the summer months which determines the plane of perpetual 
This indeed is almost an identical pipeline: for per- 
Snow is snow that lies through the heats of summer; and 
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