666 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



It will be observed that the ratio of calcium to magnesium is fairly constant 

 for all the (392) pre-Devonian analyses, in which the average is 3.35 : 1.* The 

 ratio abruptly rises in the Devonian and increases rapidly in the Carboniferous. 

 The Cretaceous shows an apparent maximum but it is quite possible that a larger 

 number of analyses of Tertiary and later formations would give average ratios 

 at least as high as that of the Cretaceous. f 



The ratio for the pre-Cambrian limestones (3.61:1 to 4.10:1), like that of 

 all the pre-Devonian, is significantly close to the ratio of calcium to magnesium 

 in the Ottawa river analyses made at the capital (low water stage, 3.82 : 1 ; 

 high water stage, 3.50:1; their average, 3.69:1). This comparison of itself 

 suggests that, during the pre-Devonian time, the river-borne magnesium and 

 calcium were wholly precipitated after diffusing to the sea bottom. In fact, the 

 correspondence must be regarded as giving powerful support to the hypothesis. 



The abrupt change in passing from the Silurian to the Devonian may, 

 perhaps, be referred to the development of the fishes during the early Devonian. 

 This development doubtless began in relatively shallow water, and the flesh- 

 eating and scavenging fishes must have aided greatly in preventing the decay 

 of animal matter on the bottom of the extensive Devonian epicontinental seas. 

 During the Carboniferous and yet more wholesale Permian and post-Permian 

 emergence the fishes were driven out into deeper water, where they continued the 

 gradual colonization of the entire sea floor. So far as the fishes are concerned 

 that colonization may have been complete in Cretaceous time4 That, at any 



* On account of the relatively small amount of time which could he occupied in 

 compiling the data, the comparison of the limestones has been largely confined to 

 the North American formations. A more qualitative study seems, however, to 

 show that there has been a parallel succession of chemical types among the lime- 

 stones of the other continents. One specially interesting parallel may be noted. 

 The Great Dolomite of South Africa (part of the Potchefstroom series of the Trans- 

 vaal) reaches a maximum thickness of 5,000 feet and covers very large areas. It 

 is unfossiliferous, but is known to be of pre-Devonian age. In chemical composi- 

 tion, grain, structure, and great thickness this huge (marine) deposit seems to be 

 very similar to the Siyeh limestone. The weathered outcrop has characteristically 

 a ' curiously wrinkled or corrugated surface, resembling an elephant's skin ' ; hence 

 the Dutch settlers have named the limestone ' Olif antsklip ' (elephant rock). This 

 corrugation is due to the more rapid solution of the calcareous parts of the rock 

 as compared with the more magnesian parts, and is thus comparable to the rough- 

 ening of the weathering surface of the Siyeh molar-tooth limestone. The average 

 content of magnesia in the whole South African formation seems to be of the same 

 order as that in the average pre-Devonian limestone of North America. See the 

 ' Geology of South Africa ' by ¥. H. Hatch and G. S. Corstorphine, London, 1905 

 p. 311. 



t Cf . C. R. Van Hise. Treatise on Metamorphism, 1904, p. 801, and Chamberlin and 

 Salisbury, Geology, Vol. 1, 1904, pp. 360, 404. 



I .Jhis speculation regarding the migration of the fishes into bathybial and 

 abyssal depths is confessedly little better than a guess, but it is stated . partly to 

 render the hypothesis somewhat more concrete and therefore more intelligible. 

 Meagre as are the relevant facts concerning the fishes, those bearing on the Paleo- 

 zoic and Mesozoic history of the bathybial and abyssal crustaceans, echinoderms, 

 worms, and other scavenging species are almost nil. The profound mystery cover- 

 ing this subject does not, however, affect the general hypothesis favouring a nearly 

 limeless ocean in pre-Cambrian time ; for it is next to certain that the more efficient 

 scavengers of the sea floor, being all relatively high types, were not abundantly 

 developed in Cambrian and pre-Cambrian time. 



