644 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



tions throughout the world. The hypothesis also accounts for the development of 

 the dolomites and limestones of pre-Silurian age. The explanation is necessarily 

 to some extent founded on speculation. The difficulty and importance of the 

 problems demand that even such speculative explanations should be retained 

 and elaborated before the final theory is adopted. For this reason the present 

 chapter contains a statement of the hypothesis in some detail. 



The secretion of calcareous hard parts by marine organisms is supposed to 

 have been first made possible as a result of the increase of the land areas during 

 the late-Huronian orogenic revolution. (See two preliminary papers.)* That 

 enlargement of the continents caused a great increase in the annual supply of 

 river-borne salts to the ocean. The supply was specially increased by the upturn- 

 ing and erosion of the thick limestones which had been deposited on the sea floor 

 of earlier pre-Cambrian time. These limestones are regarded, on the hypothesis, 

 -as precipitates of calcium and magnesian carbonates, thrown down when the 

 i"iver-borne salts diffused to the ancient sea bottom. The chief reagent for the 

 precipitation is considered to be the ammonium carbonate generated by the 

 -decay of animal matter. It is further postulated that in pre-Cambrian time 

 the active scavenging system had not yet been evolved ; that therefore the amount 

 of decaying animal matter on the pre-Cambrian sea floor was vastly greater than 

 the amount now allowed to decay on the bottom of the ocean. The smallness of 

 the annual supply of river-borne calcium salts, coupled with this specially rapid 

 precipitation of calcium carbonate, is supposed to have kept the pre-Huronian 

 ocean nearly limeless; only the minute traces of calcium salts contained in the 

 river waters as they diffused to the sea bottom would be found in the ocean of 

 that time. At the bottom the water would be practically limeless. 



The nearly limeless condition of the. surface water was changed by the 

 extensive orogenic and epeirogenic movements of late-Huronian time. In the 

 Cambrian period the animal species had begun to armour themselves with the 

 new material, henceforth present in the sea-water in sufficient amount. ' The 

 primitive chitinous shell now became strengthened .with phosphate and carbonate 

 of calcium, and in the Ordovician many species had adopted the armour or 

 skeleton of pure calcium carbonate. .The Ordovician. and Silurian rocks were 

 therefore the first to be charged with calcium carbonate shells and skeletons in 

 great numbers. 



The hypothesis further states that not only a large part, if not all, of the 

 pre-Cambrian limestones and dolomites, but, as well, the limestones and dolo- 

 mites of the early Paleozoic formations, are chemical precipitates thrown down 

 by ammonium carbonate. This precipitation grew slower in proportion to the 

 development of the fishes and other efficient bottom scavengers. When the 

 scavenging system became well established, calcium salts could, for the first time, 

 accumulate in the ocean water in excess of the needs of lime-secreting organisms. 

 Thereafter the marine limestones have been largely formed from the debris of 

 the hard parts of animals and plants. 



* R. A. Daly, Amer. Jour. Sci. Vol. 23. 1907, p. 93; Bull. Geol. Soc. America. Vol. 

 20, 1909, p. 153. 



