20 WORLD-MAKING 



Was not this a fit period for the first appearance of Hfe? 

 should we not expect it to appear, independently of the evidence 

 of the fact, so soon at least as the temperature of the ocean falls 

 sufficiently low to permit its existence ? ^ I do not propose to 

 enter here into that evidence. This we shall have occasion 

 to consider in the sequel. I would merely say here that 

 we should bear in mind that in this latter half of the Lower 

 Laurentian, or if we so choose to style it. Middle Laurentian 

 period, we have the conditions required for life in the sea 

 and on the land ; and since in other periods we know that life 

 was always present when its conditions were present, it is not 

 unreasonable to look for the earliest traces of life in this forma- 

 tion, in which we find, for the first time, the completion ot 

 those physical arrangements which make life, in such forms of 

 it as exist in the sea, possible. 



This is also a proper place to say something of the disputed 

 doctrine of what is termed metamorphism, or the chemical 

 and molecular changes which old rocks have undergone. 



The Laurentian rocks are undoubtedly greatly changed from 

 their original state, more especially in the matters of crystalli- 

 zation and the formation of disseminated minerals, by the action 

 of heat and heated water. Sandstones have thus passed into 

 quartzites, clays into slates and schists, limestones into mar- 

 bles. So far, metamorphism is not a doubtful question ; but 

 when theories of metamorphism go so far as to suppose an 

 actual change of one element for another, they go beyond the 

 bounds of chemical credibility ; yet such theories of meta- 

 morphism are often boldly advanced and made the basis of 

 important conclusions. Dr. Hunt has happily given the name 

 " metasomatosis " to this imaginary and improbable kind of 



and the acceptance of the conclusions of Nicol and Lapworth has served to 

 bring even the rocks of the Highlands of Scotland more into line with 

 those of Canada. 



^ Dana states this at i8o° F. for plants and 120° for animals. 



