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Show [PARI' I. 158 ISLAND LIFF · ~----------------~-- w- ould lower t h e h 1 by the quantity of water abstracteJ w o e ocean . from . h'l t f perfect synchromsm between the It w I e any wan o decrease' of the I.C e a t the two poles would cau.s e a m. ovement of the centre of gravity of the earth, and a shght nse .of the sea-1e ve 1 a t one p Ole and depression at .t he oth. er. .I t IS also generally believed that a great accumul~twn of ICe might cause subsidence by its pressure on the flexible crust of the .earth, and we thus have a very complex series of agents leadmg to elevations and subsidences of limited amount, such as. seern always to have accompanied glaciation. This complexi~Y of tl1 e caus es at work may explain the somewhat contr.a diC. to. ry evidence as to rise and fall of land, some authors mamtammg that it stood higher, and others lower, during the glacial perio~ . 21he state of the Planet Mars, as b~ar~ng on. the Theo1·y of .Ex centricity as a cause of Glacial P erwds.-It IS wel.l kno':n that the polar regions of the planet Mars are covered. with wh~te patches or discs, which undergo considerable alteratiOns, of size according as they are more or less exposed to the suns rays. s Lat. 20 000 with far more extensive plateaus, produce no icc-fields. We cannot: the~·efore, believe that a few thousand fee 1 t of additional£ ele~a· tion even if it occurred so recently as indicated by t 1e presenc~ o stnation~, would have produced the remarkable amount of ~laciation above described· while from the analogy of the northern hemisphere, we may well be1ie~e that it was mainly due to the same high excentricity that led to the o-laciation of Western and Central Europe, and E astern N ortlt Ameri ca. These observations confirm those of Mr. G. W. Stow, who, in a paper published in the Qtta1·terly J ourna~ of the Geological ~ocie ty (Vol. xxvii. p. 539), describes similar phenomena m th~ same mountams, and al~o mounds and ridges of unstratified clay packed ~1th ang~1a~· boulders ! wlnle ~u rt~1er south the Stormberg mountains are said to be snmlarly glacu1tcd, with Immense accumulations of morainic matter in aU the valleys. We have here all the chief surface phenomena characteristic of a glaciated country only a few degrees south of the tropic ; and taken in connection .with the ~v idence of Professor Hartt, who describes true moraines n ear H10 de Janeu o, situated on the tropic itself, we can hardly doubt the occurrence of some general and wide-spread cause of glaciation in the southern hemisphere at a period so recent that the superficial phenomena are as well preserved ~s in Europe. Such evidences of recent glaciation in the southern hemisphere are quite inexplicable without calling in the aid of the recent phase. of high excentricity ; and they may be fairly claimed as adding another lmk to the long chain of argument in favour of the tl1eory l1ere advocated. CriAI'. VIIT.l 'rll!i3 CAUSES 01!' GLACIAL EI'OCfl ~. 159 !hey have therefore been generally considered to be snow or I?e-caps, a~d to prove that Mars is now undergoing something hke a glacial period. It must always be remembered however that we are very. ignorant of the exact physical co~ditions of t?e surface of Mars. It appears to have a cloudy atmosphere hke our ~wn, but the gaseous composition of that atmosphere rna! 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 ?louds, and ~he h~at of the sun may consequently be modified m a way qmte different from anythinO' that obtains upon our ' b earth. Bearing these difficulties and uncertainties in mind let us see what are the actual facts connected with the supp~sed polar snows of Mars. 1 Mars offers an excellent subject for comparison with the Ea~th as rega.rds this question, because its excentricity is now a httle 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 inclination 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 ~hat th~y ~ow have the winter .of their northern hemispheres In penhelwn, that of their southern hemisphere beinO' in aphelion. If, therefore, the physical condition of Mars ~ere the same or nearly the same as that of the Earth, all c1rcumstances combine, according to Mr. Croll's hypothes]s, 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 ~ matter of fact Mars has two snow-caps, of nearly equal magmtude 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 pla~et are tak~n from a paper by Mr. Edward Carpenter, M.A., in the Geolog~cal Mag~z~ne of M~rch, ~877, entitled, "Evidence afforded by Mars on the subJect of Glacial Penods," but I arrive at somewhat different conclusions from those of the writer o£ the paper. |