170 R. A. DALY THE EVOLUTION OF THE LIMESTONES 



accumulate not earlier than the Devonian period. The calcium did not 

 begin to accumulate in similar excess until the general scavenging system 

 was established in the tff bathybiaF (not "abyssal") regions of the ocean 

 floor — perhaps as late as the Cretaceous period. When we also bear in 

 mind that the sodium and potassium salts have been slowly accumulating 

 from the pre-Cambrian to the present time, we are prepared to reach the 

 rather probable conclusion that the pre-Cambrian ocean really approx- 

 imated a fresh-water (though, perhaps, faintly acid) condition. The 

 only escape from that conclusion seems to be offered in the view that a 

 large part of the existing ocean is made of nearly pure "juvenile" water 

 emitted from volcanic vents or from primary igneous rocks since the pre- 

 Cambrian. 



The actual calculation of about 900 typical analyses confirms the pre- 

 vailing view that the Paleozoic and pre-Paleozoic limestones are more 

 highly magnesian than the more recent limestones. The ratio of calcium 

 to magnesium is nearly constant in the average limestones of the pre- 

 Cambrian, Cambrian, Ordovician, and Silurian formations. That ratio 

 rises abruptly in the average Devonian limestone and increases again 

 greatly in the average Carboniferous limestone. In the Cretaceous lime- 

 stones it reaches a maximum value which is very close to, or sensibly 

 equal to, that characteristic of the average Tertiary and Eecent lime- 

 stones. 



Detailed field work and microscopic and chemical study have indicated 

 that the higher proportions of magnesium in the older limestones can not 

 be explained by their having been more deeply buried and more meta- 

 morphosed than the younger limestones. The evidence shows that the 

 magnesian content of the staple pre-Devonian limestone is original, in the 

 sense that the magnesium carbonate was precipitated from sea-water. In 

 many, if not all, cases the dolomite crystals may have been formed at or 

 near the surface of the ancient calcareous muds by the interaction of the 

 magnesian salts of the sea-water with the more easily precipitated calcium 

 carbonate. Porosity of the sea bottom would aid this process, as it is 

 today favoring the dolomitization of certain more porous beds in the 

 Funafuti atoll. 



In brief, the chemical composition of the ocean water, the conditions 

 of life in the sea, and the marine limestones in general have all had a 

 correlative evolution. The hypothesis founded on this central thought is 

 at many points in this paper strongly charged with speculation; each 

 item of speculation is offered not only as a means of intelligently group- 

 ing the many facts relating to this important theme, but also, and more 

 especially, as an advertisement calling for new facts. 



