REPORT OF THE CHIEF ASTRONOMER 657 



SESSIONAL PAPER No. 25a 



the open sea opposite the greater river deltas, or such as the epicontinental seas 

 more or less isolated during the orogenic revolution. The slow spread of the 

 scavenging system may already have had some effect in the late pre-Cambrian, 

 thus increasing the chances that some calcium could remain in the oceanic 

 solution. 



Since Lower Cambrian time the continents have in part undergone submer- 

 gence and emergence, but they have doubtless never resumed their small total 

 area characteristic of the pre-Huronian period. In any case we have obvious 

 proofs that the ocean has, since the Cambrian, contained enough calcium for the 

 needs of lime-secreting organisms, and the natural explanation is to be found 

 in river inflow. 



The invention of chitinous exoskeletons (which, themselves, in Cambrian 

 types, contain some lime carbonate or phosphate and were preserved for that 

 reason) furnishes the link between the soft-bodied Eozoic animals and the post- 

 Cambrian dominant species armoured with calcium carbonate. The Cambrian 

 brachiopod shells are often similarly chitinous and offer other illustrations of the 

 link between these two principal organic epochs.* The unique and permanent 

 change in the oceanic composition made possible the dominance of post-Cam- 

 brian molluscs, brachiopods, etc., and made also possible the preservation 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 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 fossils 

 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 medusae in rocks 

 as old as the Lower Cambrian strengthens the suspicion that the metamorphic 

 hypothesis cannot explain the absence of calcareous shells or of their impressions 

 in many thousands of feet of equally little metamorphosed Eozoic sediments. 

 The impression 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. 



In passing, it may be remarked that one fundamental idea of the hypothesis, 

 namely, the variation of the amounts of calcium salts dissolved in the ocean 

 in different geological periods, may possibly be of use in helping to explain the 

 rise, culmination, and extinction of certain faunal groups. For example, the 

 immense development of the ammonites both in numbers and in their unequalled 



* J. D. Dana, Manual of Geology, p. 486, 1895. 



