THE BEGINNING OF LIFE ON THE EARTH. 25 



Dr. Sterry Hunt has pointed out in detail, that they present 

 mineral characters which show a mode of deposition different 

 from that which has prevailed subsequently, and probably 

 indicating great ejections of heated mineral matter into the 

 primitive ocean, and comparatively little of that deposit 

 therein of mere sand and clay which has prevailed in sub- 

 sequent geological periods. In short, these rocks have an 

 unmistakably primitive aspect, distinguishing them from those 

 of later times, and conveying the impression that they approach 

 at least to the records of that time when a heated ocean first 

 rested on the thin and recently solidified crust of our planet. 

 If this is really the case, then our Lower Laurentian — hard, 

 compact, destitute of limestone, and composed of material 

 which may be little else than the debrh of products of internal 

 heat merely spread out into bedded forms by water — may 

 represent a time when no living thing as yet tenanted the 

 waters ; and the dawn of life may have appeared in that period 

 when the Middle Laurentian beds were laid down. Here at 

 least we find two kinds of evidence pointing to the existence 

 of certain forms of life in the waters. 



The first depends on the mineral character of the beds 

 themselves. This formation holds several very thick beds of 

 limestone. Now although this kind of rock may, under certain 

 circumstances, 1: i deposited directly from solution in water, it 

 is not ordinarily so deposited, but more usually through the 

 agency of living beings inhabiting the waters, and forming 

 their skeletons or hard parts of limestone derived from the 

 water, usually through the medium of humble forms of plant 

 life. In this way are formed reefs of coral and beds of she'ls 

 and of chalky ooze, all composed of material once constituting 

 the skeletons of animals. The study of limestones of all 

 geological ages shows that this has been the usual mode of 

 their formation. If the Laurentian limestones had a similar 

 origin, the seas of that period must have swarmed with animals 

 having calcareous coverings ; and the study of more modern 



