648 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



possibility of abundant pre-Cambrian marine life in the form of soft-bodied, 

 highly diversified animal types.* 



The Eozoic seon was, then, divided into two parts, a long period during 

 which the calcium salts inherited from the Azoic sea were being precipitated, 

 and a much longer period during which the steady evolution of animal types took 

 place in an almost limeless sea. 



Duration of the Nearly Limeless Sea. 



The conditions suitable for the development of lime-secreting organisms 

 might have been established in three different ways. 



Putrefaction on the sea floor has, among its other effects, the generation of 

 much sulphuretted hydrogen by the decomposition of sulphates. The bottom of 

 the Eozoic ocean may have thus been poisoned by the gas in a manner similar to 

 that observed in the world's largest perfect desert, the basin of the Black Sea. 

 The evolution of bottom scavengers or at least the colonization of the general 

 sea bottom, may have been long delayed. Nevertheless, it is possible that the 

 emanation of sulphuretted hydrogen from sea water in which calcium sulphate 

 was almost entirely removed (leaving magnesium sulphate and other sulphates 

 acted on by decaying animal matter as one source of the gas) grew less as time 

 went on, and that the sea-bottom water thereby became gradually sweetened and 

 fit for colonization. The scavenging system once established, it would now be 

 possible for river-borne calcium salts to accumulate in the sea. 



Secondly, it is conceivable that the ancient animal types could elaborate 

 limey structures from even the minute quantity of calcium carbonate which 

 sea water can hold in solution, and that these animals did not then need the 

 sulphate or chloride of calcium for the secretion of calcareous structures. Cal- 

 cium carbonate could not reenter the essential composition of the ocean until the 

 acid radicals freed from the sulphate and chloride (inherited from the Azoic 

 sea) were either destroyed as such or were satisfied by yet stronger bases than 

 lime. The sulphuric acid of the existing seas is* being constantly converted into 

 insoluble iron sulphide and free sulphur. This reaction takes place best where 

 ferruginous muds are suspended in the water. It would have but limited effects 

 on the floor of the deep sea far from the pre-Cambriaji land. Nevertheless, the 

 whole water-body would, through diffusion and marine currents, be in time 

 affected by the reaction and the sulphuric acid radical of the Eozoic sea would 



* At many points in this chapter there is need for a short term designating the 

 entire pre-Paleozoic aeon of life-history on the earth. We have no generally accepted 

 word with this meaning. The writer will, accordingly, revert to the term ' Eozoic,' 

 invented nearly forty years ago by Sir J. W. Dawson and later used by him practi- 

 cally to cover the period in question. The term is here employed, however, not in a 

 stratigraphic sense, implying a division of geological time of the same order as the 

 Paleozoic, Mesozoic, etc. It is conceivable that in the future this term may be finally 

 adopted, along with ' Proterozoic ' and perhaps, other names, to represent one of 

 several ' zoic ' divisions of pre-Cambrian time. With this understanding it is hoped 

 that the proposed temporary use of the term 'Eozoic' will occasion no misapprehen- 

 sion. The pre-Eozoic seon of earth-history will be referred to as the ' Azoic' 



