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CHAP. VIII.] 
THE CAUSES OF GLACIAL EPOCHS. 
They have therefore been generally considered to be snow or 
ice-caps, and to prove that Mars is now undergoing something 
like a glacial period. It must always be remembered, however, 
that we are very ignorant of the exact physical conditions of 
the surface of Mars. It appears to have a cloudy atmosphere 
like our own, but the gaseous composition of that atmosphere 
may be different, and the clouds may be formed of other matter 
besides aqueous vapour. Its much smaller mass and attractive 
power must have an effect on the nature and extent of these 
clouds, and the heat of the sun may consequently be modified 
in a way quite different from anything that obtains upon our 
earth. Bearing these difficulties and uncertainties in mind, let 
us see what are the actual facts connected with the supposed 
polar snows of Mars. 1 
Mars offers an excellent subject for comparison with the 
Earth as regards this question, because its excentricity is now 
a little greater than the maximum excentricity of the Earth 
during the last million years, — (Mars excentricity 0.0931, 
Earth excentricity, 850,000 years back, 0.0707) ; the inclina- 
tion of its axis is also a little greater than ours (Mars 28° 51' , 
Earth 23°, 27'), and both Mars and the Earth are so situated 
that they now have the winter of their northern hemispheres 
in perihelion, that of their southern hemisphere being in 
aphelion. If, therefore, the physical condition of Mars were 
the same or nearly the same as that of the Earth, all cir- 
cumstances combine, according to Mr. Croll’s hypothesis, to 
produce a severe glacial epoch in its southern, with a perpetual 
spring or summer in its northern, hemisphere ; while on the 
hypothesis here advocated we should expect glaciation at both 
poles. As a matter of fact Mars has two snow-caps, of nearly 
equal magnitude at their maximum in winter, but varying very 
unequally. The northern cap varies slowly and little, the 
southern varies rapidly and largely. 
1 The astronomical facts connected with the motions and appearance of 
the planet are taken from a paper by Mr. Edward Carpenter, M.A., in the 
Geological Magazine of March, 1877, entitled, “Evidence afforded by 
Mars on the subject of Glacial Periods,” but I arrive at somewhat different 
conclusions from those of the writer of the paper. 
