458 J. D. Dana — Subdivisions in Archaean History. 



The beds of the upper Archaean, under the conditions existing, 

 may well, over some regions, be uncrystalline still, and may 

 include carbonaceous shales that hold to this time their car- 

 bonaceous products. Such uncrystalline beds may now exist 

 over the Continental Interior; for the great Interior has 

 generally escaped when metamorphic work was in progress on 

 the Continental borders. 



The amount of carbonic acid is most readily estimated by first 

 obtaining the probable amount for all post-Archoean sources, and 

 then adding to this that which is indicated by Archrean terranes. 

 The calculation is here given in detail that others may use it for 

 deductions from other estimates. 



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 T Vth 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 one foot thick 

 would give to the atmosphere, 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=l*5, will give to the atmosphere by oxidation 19 

 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 -r^th 

 impurity, is 1000 feet, the contained carbonic acid amounts accord- 

 ing to the above to 500 pounds per square inch, or 34 atmospheres 

 (of 14f pounds), and if the mean thickness of the coal is one foot, 

 the carbouic acid it could contribute would be 1*9 pounds per 

 square inch. Adding these amounts to the carbonic acid cor- 

 responding to the carbon in the mineral oil and gas and other car- 

 bonaceous products of the rocks and organic life, supposing it 

 to be six times that of the coal, the total is 513*5 pounds, or 35 

 atmospheres. The mean thickness of Archaean calcium, magne- 

 sium and iron carbonates is not a fourth of that of post-Archaean. 

 Estimating the carbonic acid they contain and that corresponding 

 to the graphite of the rocks at ten atmospheres, the whole amount 

 becomes 45 atmospheres. 



To bring the amount up to the estimate for early Archaean 

 time of 200 atmospheres of carbonic acid, the mean thickness of 

 the limestone for Archaean and post- Archaean time should be taken 

 at nearly 6000 feet. 



Part of the limestone of post-Archaean terranes was derived 

 from the wear and solution of Archaean limestones, iron carbon- 

 ate, etc., and hence all the 35 atmospheres to the square inch 

 were not in the atmosphere at the commencement of the Paleo- 

 zoic. But if we reduce the 35 atmospheres on this account, to 25 



