232 Pi'of, Hughes, Criticism of the Geological [Oct. 30, 



period when the conditions for migration were somewhat more 

 favourable than they are now." 



Another argument for the elevation, more or less recent, of the 

 eastern part of Africa and of the great chain of islands which 

 connect Australia and the south-eastern promontories of Asia is 

 that the dividing hollows between these islands themselves and 

 between them and the mainland seem to be chiefly valleys of 

 subaerial denudation, and to bring the bottom of the deepest 

 valley up to the surface would imply an upheaval of at least 

 15,000 feet. 



The same kind of evidence as that founded on the thickness 

 and known movements up and down of the sedimentary rocks in 

 the Atlantic basin is forthcoming also in the basin of the Indian 

 Ocean. The Jurassic rocks of many parts of India and Australia 

 have been estimated at from 6000 to 10,000 feet, and the Basic 

 rocks of Cretaceous age in the Deccan amount in the aggregate 

 to 6000 feet. Series referred to the Tertiary period attain a 

 thickness of 10,000 feet, but, more important for our present 

 purpose. Tertiary beds have been lifted up in the Himalayas to 

 more than 16,000 feet above sea-level. 



Effect of Elevation on Climate. 



We have now to consider what would be the effect on tem- 

 perature and precipitation of such earth movements as those 

 which we certainly know did repeatedly occur round the areas just 

 described. 



First we must bear in mind that it is not necessary to have 

 very great cold in order to obtain glacial conditions. All that 

 is required is, that the temperature shall be below freezing- 

 point. Great masses of ice encroaching on the lowlands and 

 icebergs drifting into temperate regions maintain a low temperature 

 around themselves at the expense of their mass. All that is wanted 

 is a large collecting ground and abundant precipitation to keep 

 up the supply. If the cold is excessive it arrests the moisture 

 before it reaches the neve. 



Now it is a well-known fact that as we climb the higher 

 mountains in any part of the world the cold increases, and the 

 forms of life proper to more northern regions appear succes- 

 sively along the belts where they find the temperature that suits 

 them. The rate of increment is not quite constant, but a fall of 

 1° F. for every 250 to 300 feet additional height is about the aver- 

 age. If then we start from the mountain foot in tropical regions it 

 is long before we reach the freezing isotherm, in northern climates 

 we reach it sooner; but sooner or later it must be found if the 

 mountains rise to a sufficient height. In high northern latitudes 



