PALEOZOIC TIME — CAMBRIAN. 485 



purity attained, is unknown ; experiments on modern life may possibly lead, 

 to some reasonable estimate. This much is certain: that the waters were 

 sufficiently pure for the development of a great diversity, as has been shown, 

 of aquatic life. The types of the early Cambrian are mostly identical with 

 those now represented in existing seas, and. although inferior in general as 

 to i-vade, they bear no marks of imperfect or stunted growth from unfit or 

 foul surroundings. How the purification was made so complete by the 

 beginning of Paleozoic time has not been explained. 



The following observations have an important bearing on this subject, although falling 

 short of the needed explanation : — 



If the carbonic acid in the limestones of the world and other carbonates, and the carbon 

 of the coal and carbonaceous products in the rocks were originally, as is believed, in the air 

 and waters, the amount of these carbonates and carbonaceous products in the formations 

 of the Cambrian and all later periods would afford a basis for estimating approximately 

 the amount of available carbonic acid existing at the beginning of these periods. 



For the estimation there are the following data. A cubic foot of pure limestone 

 which is half calcite and half dolomite, and has the normal specific gravity 2-75, weighs 

 171-4 pounds ; and this, allowing for yV impurity, becomes 157 pounds, and corresponds 

 to 72 pounds of carbonic acid. A cubic foot is equal to an inch-square column 144 feet in 

 height. Since 72 is half of 144, each foot of the column of such limestone contains half a 

 pound of carbonic acid. Hence a layer of the limestone 1 foot thick would give to the 

 atn osphere, on decomposition, half a pound of carbonic acid for each square inch of 

 surface. 



A foot layer of good bituminous coal containing 80 per cent of carbon, G = 1-5, will 

 give to the atmosphere, by oxidation, 1-9 pounds of carbonic acid per square inch of 

 surface. 



If the mean thickness of the limestone over the whole earth's surface, that of the 

 oceans included, reckoned on a basis of J, impurity, is 1000 feet, the contained carbonic 

 acid amounts, according to the above, to 500 pounds per square inch, or 34 atmospheres 

 (of 14| pounds), and if the mean thickness of the coal is 1 foot, the carbonic acid it 

 could contribute would be 1-9 pounds per square inch. Adding these amounts to the 

 carbonic acid corresponding to the carbon in the mineral oil and gas and other carbona- 

 ceous products of the rocks and organic life, supposing it to be 6 times that of the coal, 

 the total is 513-5 pounds, or 35 atmospheres. The mean thickness of Archaean calcium, 

 magnesium, and iron carbonate is not a fourth of that of post-Archsean. Estimating the 

 carbonic acid they contain and that corresponding to the graphite of the rocks at 10 

 atmospheres, the whole amount becomes 45 atmospheres. 



It has been suggested by some writers that the total amount of carbonic acid in the 

 early Archaean was equivalent in pressure to 200 atmospheres. But this would require 

 that the mean thickness of the limestone for Archaean and post-Archaean time should 

 have been nearly 6000 feet. 



Part of the limestone of post-Archsean terranes was derived from the wear and solu- 

 tion of Archaean limestones, iron carbonate, etc. , and hence all the 35 atmospheres to the 

 square inch were not in the atmosphere at the commencement of the Paleozoic. But if 

 we reduce the 35 atmospheres, on this account, to 25 atmospheres, it is still an enormous 

 amount beyond what ordinary life, even aquatic life, will endure. Reducing the esti- 

 mated mean thickness for the limestone layer over the globe from 1000 to 500 feet would 

 make the amount less by nearly one half. But with all the reductions that can be explained, 

 the excess is still very large. It has been proved by experiment that an excess also of 

 oxygen diminishes the deleterious influence of carbonic acid on plants ; and that if the 



