2 3 o THE POPULAR SCIENCE MONTHLY. 



nificant change. It is true we have still a predominance of gneisses 

 which may have been formed in the same manner with those below 

 them ; but we find these now associated with great beds of limestone 

 and dolomite, which must have been formed by the separation of cal- 

 cium and magnesium carbonates from the sea-water, either by chemical 

 precipitation or by the agency of living beings. We have also quartz- 

 ite, quartzose gneisses, and even pebble-beds, which inform us of 

 sand-banks and shores. Nay, more, we have beds containing graphite 

 which must be the residue of plants, and iron-ores which tell of the 

 deoxidation of iron oxide by organic matters. In short, here we have 

 evidence of new factors in world-building of land and ocean, of at- 

 mospheric decay of rocks, of deoxidizing processes carried on by vege- 

 table life on the land and in the waters, of limestone-building in the 

 sea. To afford material for such rocks, the old Ottawa gneiss must 

 have been lifted up into continents and mountain-masses. Under the 

 slow but sure action of the carbonic dioxide dissolved in rain-water, 

 its feldspar had crumbled down in the course of ages. Its potash, soda, 

 lime, magnesia, and part of its silica, had been washed into the sea, 

 there to enter into new combinations and to form new deposits. The 

 crumbling residue of fine clay and sand had been also washed down 

 into the borders of the ocean, and had been there deposited in beds.* 

 Thus the earth had entered into a new phase, which continues onward 

 through the geological ages ; and I place in your hands one key for 

 unlocking the mystery of the world when I affirm that this great 

 change took place, this new era was inaugurated, in the midst of the 

 Laurentian period. 



"Was not this time a fit period for the first appearance of life ? 

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

 have of the fact? I do not propose to enter here into that evidence, 

 more especially in the case of the one well-characterized Laurentian 

 fossil, Eozoon Canadense. I have already amply illustrated it else- 

 where. 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 unrea- 

 sonable to look for the first traces of life in this formation, in which 

 we find for the first time the completion of those physical arrange- 

 ments which make life, in such forms of it as exist on our planet, 

 possible. 



This is also a proper place to say something of the doctrine of what 

 is termed "metamorphism." The Laurentian rocks are undoubtedly 

 greatly changed from their original state, more especially in the mat- 

 ters of crystallization and the formation of disseminated minerals by 



* Dr. Hunt has now in preparation for the press an important paper on this subject, 

 read before the National Academy of Sciences. 



