606 THE GEOLOGICAL RELATIONS OF THE ATMOSPHERE. 



T. Hardman, F. C. S., hiis a very exhaustive paper on "The Atmosphere 

 Considered in its G-eological Relations," from whi('h we extraet the follow- 

 ing interesting facts : 



Perfectly pure water has a very appreciable solvent effect on rocks. 

 which is immensely augmented when it is chemically chai-ged with car- 

 bonic acid, oxygen, nitric acid, and other matters derived directly or indi- 

 rectly from the atmosphere. But while on the one hand the influence ot 

 the atmosphere disintegrates and destroys rock masses, on the other it is 

 mighty in building them up. Without the small percentage of carbonic 

 acid contained in the air there could be no vegetation, and there would be 

 none of the coal beds which form such important members of our rock for- 

 mations. The immense masses of limestone found everywhere, and the 

 coral reef of the present day, must owe their being indirectly to the car- 

 bonic acid of former atmospheres. A drop of rain water absorbs a trace of 

 carbonic acid from the atmosphere, falls on a rock containing lime in some 

 form, dissolves the lime as bicarbonate, carries it down to the ocean, and 

 finally gives it up to become part of the skeleton of a coral or mollusc, 

 which in its turn may form a portion of an immense mass of limestone 

 rock. 



The bulk of the atmosphere is made up of oxygen and nitrogen, but 

 these do not take so active shares in geological matters as the almost in- 

 finitesimal trace of carbonic acid present. The amount ranges from 3 to 10 

 volumes in 10,000 volumes of air. The principal sources of increase are. 

 volcanic and other subterranean exhalations; respiration of animals; com- 

 bustion of fuel and vegetable decay. 



The series of rock-metamorphisms due to the simple absorption of car- 

 bonic acid b}' a plant is very interesting. The carbon is assimilated by the 

 plant, and it dies and becomes thus a part of a coal bed or lies embedded 

 in sediment of some kind. Decomposition sets in ; and if there be a redu- 

 cible compound near it, chemical changes result. If the strata contains 

 sulphate of iron, it is reduced to sulphide, commorJ}" known as iron pyrites 

 or false gold. The reduction is effected by the carbon of the plant abstract- 

 ing the oxygen from the sulphate. The resulting carbonic acid either is 

 taken up hy percolating water and penetrates farther into the heart of the 

 rock, effecting new changes, or it finds its way to the surface through some 

 crevice, or by aid of a mineral spring, and once more miugles with the at- 

 mosphere, to be perhaps again absorbed by vegetation and pass through a 

 similar round of changes afresh. In many cases the action of the carbonic 

 acid changes a metallic ore from an insoluble to a soluble compound, thus 

 reducing the ancient crystalline rocks. The metals carried away by streams 

 were deposited along their beds, and valuable beds of ore were formed. 



The atmosphere in the carboniferous age contained a much larger por- 

 tion of carbonic acid. This has been gradually absorbed into the earth, until 

 the amount stored in the earth is estimated at 6,620 times as much as there 

 is in the atmosj)here. although the' latter contains 1,250,000,000,000 tons of 



