100 R. A. Daly — Timeless Ocean of P re-Cambrian Time. 



in solution, to the sea. At least three conditions were present 

 to favor at this time a special enrichment of the sea-water in 

 soluble salts of calcium. Great volumes of basic volcanic 

 rocks were now for the first time exposed to weathering, with 

 the necessary evolution of lime salts ; the limestones chemically 

 precipitated in Azoic and earlier Eozoic periods were now, 

 for the first time, exposed to solution in rain-water ; and 

 the areas of the lands and of drainage-basins, with all their 

 assemblage of weathering heterogeneous rocks, were probably 

 greatly increased over their magnitudes in former times. 



In view of the slowness of diffusion in a body so great as the 

 ocean, it is not difficult to believe that this special influx of 

 river-borne calcium salts might keep the surface layers of the 

 sea-water sufficiently supplied with calcium sulphate and 

 carbonate for organic needs while the bottom stratum was as 

 continuously being depleted of all lime salts. Since lower Cam- 

 brian time the continents have in part undergone submergence 

 and emergence, but they have doubtless never resumed their 

 small total area characteristic of the early Eozoic period. We 

 are therefore justified in believing that the river-borne calcium 

 salts have nearly as well supplied lime-secreting marine organ- 

 isms throughout post-Cambrian time. 



It is obviously impossible to decide as to which of these 

 three suppositions is the true one or as to whether all three 

 may contain some of the whole truth. As a working hypoth- 

 esis the third alternative seems the most promising. There 

 is something to be said for the view that the colonization of 

 the general sea-floor by active scavengers did not occur until 

 after Cambrian time ; without here entering on the discussion 

 of the question, this conclusion is assumed to be a fact. It 

 seems almost certain, however, that the " lime habit " of marine 

 organisms first became fully established at a time subse- 

 quent to the great pre-Cambrian orogenic revolution. We 

 assume, for further argument, that the new "lime-habit" was, 

 in the main, dependent on the preliminary secular weathering 

 of the continents which were enlarged by that revolution, and 

 enriched in the highly soluble limestones then upfolded. 



The oldest calcareous fossils in the stratified rocks. — 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 domi- 

 nant species armored 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 



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



