484 A. C. Lane — Divisions of the Pre-Camhrian. 



When life came, perhaps waiting until enough carhon for its 

 existence had accumulated in the ocean from volcanic sources, there 

 would he a gradual development of more ordinary sediments, and river 

 waters, with sodium carbonate and silicate in solution, such as now 

 drain granite areas, would begin the precipitation of calciuai chloride 

 as calcium carbonate and accumulation of sodium chloride, which 

 has been the dominant factor in oceanic chemical evolution. Very 

 promptlj-, however (this theory is due to Leith), the sodium silicate 

 would react upon the chloride of iron, precipitating the silica and iron 

 as iron oxide and chert, raised perhaps with carbonates. I have 

 recently seen an ideal specimen of such a rock from a 2,000 feet diill- 

 hole in the deep continuation of the Mesabi range. Thus we have the 

 cherty iron members, which are so universal a feature of the various 

 Huronian series, and we also have sodium chloride left in solution. 



During the period of the dawn of life there would then be as 

 characteristic the chemical precipitation of the carbonates of calcium, 

 iron, magnesium, etc., and of chert and iron oxides, from chloride or 

 sulphate solutions.^ Decaying organic matter would also produce the 

 black muds from which come the graphitic slates. The other sediments 

 would approach gradually the customary types. 



Chamberlin and Salisbury have touched upon the abnormal character 

 of the pre-Cambrian sediments, though not using it as a basis of sub- 

 division. 



During all this time, however, the ocean remained relatively fresh 

 and unfavourable to the secretion of shells, and the rapid evolution 

 of life into various branches of the animal kingdom went on without 

 hard parts, and without a body-cavity closed from the oceanic vital 

 medium. But with the steady accumulation of salts in the ocean, 

 its waters reached and passed the physiological optimum of eight parts 

 per thousand. Numerous different branches responded to this change 

 of environment for the worse by secreting calcium carbonate or 

 phosphate, which were already present to saturation, at first as a 

 pure physiological or pathological necessity like renal calculi. But 

 it at once was found to be of immense value as a skeletal support and 

 protection. Only in some such way does it appear to the writer that 

 we can account for the appearance of hard parts in numerous branches 

 of the animal kingdom at about the same time. It must be due to 

 a general reaction to some general change of environment, and this 

 particular change, supported as it is by analyses of connate waters 

 and the general drift of chemical evolution of the ocean, as well as by 

 the physiological evidence so acutely marshalled by Quinton, is by far 

 the most plausible. This event marks practically the beginning of the 

 Cambrian. The classification which the writer would suggest as 

 probably that of the future is tabulated below. The connotation is 

 very different from that of Yan Hise, but in denotation of application 

 the only changes needed will be to remove perhaps a small portion of 

 that generally referred to as Keewatin, as well as the Grenville 

 Limestone and associated beds, from the Azoic. 



1 Judging from the results of analyses of connate waters, the writer leans to 

 chlorides. 



