37 



crust increases as we descend, and, upon the whole, the rate of increase is very 

 uniform, it being somewhat more rapid in cold regions, and less so in hot ones, 

 which is what might have been expected. Upon the average, the increase is about 

 1 deg. Fahr. for every 50 feet of descent into the earth. This result must be stated 

 subject to the reservation that any depth yet explored by man is very trifling 

 when compared to the thickness of the earth's crust, and also that some of these 

 experiments at great depths were very unsatisfactory, owing to the borings 

 containing water, in which, by the virtue of the law of equilibrium of fluids, 

 currents were produced which carried the warmer and lighter water to the top, 

 and the heavier and colder water to the bottom, thus interfering with the accuracy 

 of the experiment by making the top strata too warm and the bottom strata too 

 cool. This has taken place to such an extent on some occasions, as to lead some 

 observers to conclude that after a certain depth, no increase of temperature took 

 place at all, and that the source of heat in the crust of the earth was in itself, and 

 due to chemical changes, and not due to some central internal heat, such as the 

 advocates of the theory of a constant increase of temperature are bound to believe 

 in. However, to bring these remarks to bear upon our subject of geological 

 time ; Professor Thomson has calculated that this rate of increase of temperature 

 has diminished from 1 deg. for every 10 feet of descent, to 1 deg. in every 50 feet, 

 as at present, during the last 96 millions of years, and he says that if this process 

 has been going on with anything like uniformity for 200 millions of years, the 

 amount of heat lost in that time would be sufficient to melt the rocks forming the 

 earth's crust, and that, therefore, it is improbable that life has existed upon this 

 earth for more than 100 millions of years. But here I must once more point out 

 that these calculations, interesting as they are, are only based upon assumptions 

 which are not fuUy proved, and that a very slight error of observation would 

 create a very great error in the calculations, and, at best, the results arrived at are 

 va^e and undetermined. 



I must now take a brief glance at what may, by some geologists 

 be considered as the surest and best guide for measuring geological time, 

 namely, organic remains. It has been established in palaeontology that some 

 forms of life had their origin before others, and the rocks in which certain forms of 

 animal remains, such, for example, as Trilobites are found, are supposed to be of 

 more ancient date than those which contain, say, the Numinulites, and indeed it 

 is chiefly upon the animal remains found in the different rocks, that these rocks 

 have been separated into epochs, periods, strata, and so forth. But without in 

 the least, wishing to depreciate this method of geological reasoning, or to deny its 

 truth, as far as the order of superposition of rocks is concerned, I must point out 

 in two or three ways, how difl&cult it is to derive from its source any information 

 which can really be looked upon as reliable, as to the duration of time which any 

 particular rocks took in being formed. And first we must be convinced of this 

 fact in biology, that the changes in the animal and vegetable kingdoms are chiefly 

 due to the alteration of the circumstances surrounding them, and if the circum- 

 stances remain the same, the species of animals and plants will continue the same 



