164 ISLAND LIFE 



abstracted from it, while any want of perfect synchronism 



betwjeen the decrease of the ice at the two poles would 

 cause a movement of the centre of gravity of the earth, 

 and a slight rise of the sea-level at one pole and depression 

 at the other. It is also generally believed that a great 

 accumulation of ice would cause subsidence by its pressure 

 on the flexible crust of the earth, and we thus have a very 

 complex series of agents leading to elevations and sub- 

 sidences of limited amount, such as seem always to have 

 accompanied glaciation. This complexity of the causes at 

 work may explain the somewhat contradictory evidence as 

 to rise and fall of land, some authors maintaining that 

 it stood higher, and others lower, durino: the orlacial 

 period. 



The State of the Planet Mars, as Bearing on the Theory of 

 Excentricity as a Cause of Glacial Periods. — It is well known 

 that the polar regions of the planet Mars are covered with 

 white patches or discs, which undergo considerable altera- 

 tions of size according as they are more or less exposed to 

 the sun's rays. They have therefore been generally con- 

 sidered 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 dif- 

 ferent, 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 con- 

 sequently be modified in a way quite different from any- 

 thing that obtains upon our earth. Bearing these diffi- 

 culties and uncertainties in mind, let us see what are the 

 actual facts connected with the sujDposed polar snows of 

 Mars.i 



^ 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 con- 

 clusions from those of the writer of the paper. 



