R. A. Daly — Limelesa Ocean of Pre-Cambrian Time. 101 



composition made possible the dominance of post-Cambrian 

 molluscs, brachiopods, etc., and made also possible the preserva- 

 tion of countless post-Cambrian fossils. 



Following our main hypothesis, the chief animal fossils 

 expected in Eozoic rock are impressions of soft-bodied species, 

 the tests of siliceous organisms, and chitinous tests. The last 

 will be expected only in the higher beds of the series and 

 should owe their preservation to limey ingredients secreted by 

 the animals inhabiting the late Eozoic sea. Along with the 

 chitinous fossils may be a few calcareous shells or skeletons 

 also evolved because of the late Eozoic enrichment of the sea 

 in river-borne lime salts. These are, in fact, the kinds of fos- 

 sils discovered in the pre-Cambrian rocks by Walcott, Barrois, 

 Cayeux and others. For obvious reasons fossils of all four 

 classes must be few or else difficult to discover in the rocks. 

 The very presence of the impressions of medusas in rocks as 

 old as the lower Cambrian strengthens the suspicion that the 

 metamorphic hypothesis cannot explain the absence of calcare- 

 ous shells or of their impressions in many thousands of feet of 

 equally little metamorphosed Eozoic sediments. The impres- 

 sion of a shell is assuredly more likely to be preserved in 

 mud or sand than is the impression of a medusoid animal. It 

 seems, on the other hand, certain that the pre-Cambrian rocks 

 of the North American Cordillera never at any time contained 

 any considerable number of calcareous shells or skeletons. The 

 same conclusion applies in some measure to the Cambrian 

 rocks of the British Columbia-Montana section. 



Tests of the Suggested Hypothesis. 



The rearrangement in the chemical constituents of pre-Cam- 

 brian ocean-water through the decay of animal matter is the 

 fundamental premise of the hypothesis and it deserves special 

 examination and illustration. The tests of the premise are at 

 least threefold : — laboratory experiment, observations on exist- 

 ing seas, and the witness of pre-Cambrian deposits, particularly 

 of the carbonates. 



1. Corroborative experiments. — Murray, Woodhead and 

 Irvine have made a number of valuable observations on the chem- 

 ical modification of sea-water exposed to the emanations of 

 putrefying animal matter and to the effete substances derived 

 from living animals.* 



In one experiment four small crabs weighing 90"72 grams 



were placed in sea-water absolutely free from carbonate of 



lime. After twelve months they produced an alkalinity in 



the water equal to the production of 45*36 grams of calcium 



* Proceedings, Eoyal Society of Edinburgh, vol. xvii, p. 79, 1889. 



