Hill. — Geology of Hawke's Bay. 467 



energy, for the sun, like everything else of which we are cog- 

 nisant, must have had a beginning, and he will also have an 

 end. If such is the case, it follows that the supply of heat 

 which the sun has given and is giving off is a varying 

 quantity, diminishing slowly and surely as periods roll on. It 

 may be assumed, therefore, that much more heat was given 

 off from the sun during the Pleistocene period than is given 

 off now ; hence more heat must have reached the earth in 

 each zone than is the case at the present time. It follows 

 also that, so far as the heat from the sun affects the earth, 

 the latter was, on the average, warmer in Pleistocene times 

 than in Recent. 



Now, the question of temperature is one of great import- 

 ance in connection with all aqueous and aerial movements. If 

 the sun is the source of a varying supply of heat, it is certain 

 that the earth, if the nebular theory is assumed, is also parting 

 with a varying quantity of heat. This earth cannot possibly re- 

 main at the same temperature for an indefinite period of time, 

 for, without considering the amount of heat supplied by the 

 sun, it follows from the law of exchanges that this earth is part- 

 ing with some of its own heat into outer space. It is true the 

 amount is small, being only ^$th of a degree per annum, but 

 continued for a long period of time the quantity becomes of 

 high importance. From the equator, as we proceed either in 

 vertical or horizontal space, the cold increases at a progressive 

 rate, so that on the equator at an elevation of 15,000ft., or in 

 round numbers, say, three miles, a place is reached which is 

 always below the temperature of frozen water ; and the same 

 may be said with respect to temperature beyond the arctic 

 and antarctic circles, in the direction of the poles. This con- 

 dition of things could not always have been so: in fact, the 

 temperature of the earth at one period of its history was such 

 that unless the mountains were much higher than they now are 

 no snow could have remained upon them, even towards the 

 poles, and frost and hail must have been unknown. At that time 

 the temperature of the atmosphere must have been such that 

 much more aqueous vapour was held in suspension than is now 

 possible, as the quantity of vapour is limited by the temperature 

 of the air. Thus at the present time a cubic foot of dry air 

 weighs about ljoz. ; and this at a temperature of 60° Fahr. 

 will absorb about 5^-gr. of vapour, and no more. At a tem- 

 perature of 80° Fahr. the same quantity of air will absorb about 

 llgr. of vapour, and so on according to the degree of tem- 

 perature of the atmosphere, it being found that in order to 

 saturate a given space like a cubic foot the vapour required 

 increases as the temperature increases. If, as is now recog- 

 nized by most geologists, the earth has reached its present 

 condition by a slow process of cooling, it follows that the 



