974 A TREATISE ON METAMORPHISM. 



less extent in the carbonaceous shales. The direct source of the carbon 

 for this concentration is the atmosphere, which has been continuously 

 replenished in various ways. 



The certain original sources of carbon dioxide for the replenishment 

 of the atmosphere were the magmas and meteorites. Silication, the 

 oxidation of organic material, and the ocean return to the atmosphere 

 carbon dioxide which has been taken from it. But all of the above 

 supplies of carbon dioxide are available to replenish the atmosphere. Con- 

 cluding, the chief processes which abstract carbon dioxide from the 

 atmosphere are those of carbonation and the building up of carbonaceous 

 deposits. All of the replenishing processes, including the reversing 

 processes of silication and the oxidation of buried carbon compounds, 

 have been barely able to keep a minute portion of carbon dioxide 

 in the atmosphere — 0.030 per cent by volume, or 0.045 per cent by 

 weight. It is probable, however, that the work of man, especially during 

 the last half century, has returned a great volume of carbon dioxide to the 

 atmosphere by the artificial oxidation of carbonaceous material, and thus 

 has reversed the average of the processes of nature, which plainly appear 

 to have caused depletion of the carbon dioxide in the atmosphere. In 

 consequence, at the present time the amount of carbon dioxide in the 

 atmosphere may be increasing rather than decreasing. 



TITANIUM. 



According to Clarke's estimate of 1891 titanium comprises 0.30 per 

 cent of the original rocks, the lithosphere, and the atmosphere together. 

 All of the titanium is in the original rocks, of which it composes 0.33 per 

 cent. In 1900 Clarke increased his estimate of titanium in the original 

 rocks to 0.41 per cent, thus giving it ninth place in the scale of abundance 

 next to potassium. In passing from sodium to titanium we go from the 

 elements which may be called abundant to those which are subordinate. 

 Reckoned as an oxide (Ti0 2 ) Clarke's estimate of the amount in the original 

 rocks for 1891 is 0.55 per cent and for 1900 is 0.60 per cent. 



In. the original rocks titanium occurs both as an oxide and as a titanite. 

 As an oxide it is found in ilmenite and in the rutile group. As a titanite 

 it is found in titanite and perovskite. The amount of these minerals 

 in the rocks is not usually large, but they are very widespread. The 



