104: JR. A. Daly — -Limeless Ocean of Pre- Cambrian Time. 



free sulphuretted hydrogen, but also states another cause of 

 the poverty of the bottom waters in free oxygen. It is in part 

 due to the lack of normal vertical currents in the Black Sea ; 

 these are impossible because of the peculiar density stratifica- 

 tion of this basin. 



The equations show that, in the presence of ferruginous 

 muds, free sulphur is precipitated along with the sulphide of 

 iron. It is thus easy to understand the formation of the numer- 

 ous nodules of iron pyrites found in the black muds. In 

 the main ocean near muddy shores the foregoing reactions also 

 apply to a part of the changes produced by putrefaction. The 

 analogous case of the Black Sea, therefore, proves the truth 

 of the prevailing views as to the formation of iron pyrites in 

 marine sediments, and also the corollary of our hypothesis as 

 to the gradual destruction of the sulphuric acid radical in 

 Eozoic seawater. That all the lime salts have not been pre- 

 cipitated from the Black Sea water is, of course, due to the 

 large amount of Mediterranean water constantly renewing the 

 lime salts by way of the bottom current at the Bosphorus. 



It seems clear that the Black Sea is carrying on a gigantic 

 natural experiment which strengthens belief in the main 

 deductions so far made as to the physical and biological con- 

 ditions of the Eozoic sea. In one important respect the analogy 

 breaks down ; we shall see that, after all the lime salts are 

 removed from sea-water, the ammonium carbonate has special 

 power to attack the magnesium salts. This fact cannot, for 

 the reason just stated, be illustrated in the case of the Black 

 Sea. 



3. Pre- Cambrian Sedimentary Deposits. 



A third test of the hypothesis consists in an examination of 

 actual rock-deposits in the pre-Cambrian. 



Origin of dolomite and other magnesian sediments. — It 

 is an established fact that dolomites and magnesian lime- 

 stones, sandstones and argillites are very common in pre-Cam- 

 brian rock-series. Granting that Eozoic organisms could not 

 secrete magnesium carbonate shells or skeletons, it follows that 

 the magnesian content of these rocks must have had a chemi- 

 cal origin. 



The magnesium carbonate was not thrown down simply 

 because this little soluble salt, as it was introduced by the pre- 

 Cambrian rivers, saturated the sea-water. Then as now cal- 

 cium carbonate was doubtless much in excess of magnesium 

 carbonate in river-waters. We have seen that the acid radi- 

 cals set free in the persistent precipitation of calcium carbon- 

 ate, would, during Eozoic time, prevent the permanent solution 

 of magnesium carbonate in any appreciable quantity. On the 



