177 



with Sir "William Crookos,'" picture to ourselves a time wlien the 

 matter from which our solar system has been evolved existed as a 

 vast jsea of incandescent mist, which he calls " protyle," in which 

 atoms had not yet been formed. Then came the formation of atoms 

 and the genesis of the elements. The earth and the other planets 

 of our system were in course of time thrown off as gaseous rings 

 during successive stages in the cooling process. By and by our 

 earth acquired a molten nucleus, and after long ages a solid crust 

 was formed. 



Of course at first the earth's solid exterior would be intensely 

 hot, and no water could rest in liquid form upon it. It i.s, 

 hoAvever, supposed by some competent physicists that it would 

 not be long before the crust — being a good non-conductor of 

 heat — would have so far cooled as to allow of the resting upon 

 it of water, which would of course at first have a temperature near 

 the boiling point. 



It may be a matter of opinion, Tjut I think it is probable that 

 the exterior surface of the earth was dimpled and embossed from 

 the first, owing partly at least to the unequal distribution of the 

 enormoas pressure of the atmosphere of the time — abo\it 5,000 lbs. 

 to the square inch — and by the time the Avater was able to collect 

 upon it, it had become, from differential contraction, so irregular 

 that portions of it stood above the primitive sea-level. In this 

 case rain and rivers, as well as the action of waves and tides, 

 would set to work at once upon the portions projecting above the 

 level of the water, and would tend to reduce them to that level, 

 Avhile the denuded material would be spread out under the water, 

 and woidd become assorted more or less into coarser and finer. 

 And so we get the beginning of that series of stratified deposits, 

 each entombing the organisms of its own particular epoch, which 

 has been forming unceasingly since, and which will continue to 

 form " while the earth remaineth." 



The date at which the earth's crust was formed, and the 

 earliest strata laid down, is, of course, most diificvdt to determine. 

 But various attempts have been made to solve the ])robleni. Lord 

 Kelvin, basing his arguments on the rate of increase in the 

 temperature of the crust as we go downwards, on the retardation 

 of the earth's angular velocity by ■ tidal friction, and on the 

 limitation of the age of the sini, came to the conclusion that 

 "the existing state of things on the earth, life on the earth — all 

 geological history showing continuitj- of life — must be limited 



(1) See his address to Section B of the British Association in 1886. 



