990 A TREATISE ON METAMORPHISM. 



same proportion by the processes of metamorpbism the iron-ore deposits 

 should be 66.3 times as great in magnitude as the manganese deposits, and 

 this ratio is certainly approximated by the facts. There can be no better 

 illustration of the importance of the chemical law of mass action in 

 geological processes. 



CALCIUM. 



According to Clarke's estimate of 1891 calcium composes 3.51 per cent 

 of the outer 10 miles of the crust of the earth, including the original rocks, 

 hydrosphere, and atmosphere; of the original rocks alone 3.77 per cent, and 

 of the ocean alone 0.05 per cent. According to his estimate of 1900 the 

 amount of calcium in the original rocks is 3.50 per cent." Calcium is fifth 

 in abundance among the elements. Reckoned as an oxide Clarke's estimate 

 of the amount in the original rocks was 5.29 per cent in 1891, and 4.90 per 

 cent in 1900. This gives lime fourth place in the table of oxides. 



Calcium is an abundant constituent of many of the minerals of the 

 original rocks. It occurs in all of the feldspars except the acid end of the 

 series. It is an essential constitueut of nearly all of the pyroxenes and 

 amphiboles, being, however, more abundant in the former than in the latter. 

 For instance in diopside- the ratio between the calcium and magnesium is 

 1:1, whereas in tremolite the ratio is 1 : 3. It is ail essential constituent of 

 the scapolites, meionite and wernerite, which, however, do not form an 

 especially important group of minerals in the igneous rocks. While 

 calcium occurs in fewer minerals than magnesium and is less abundant 

 than magnesium in the pyroxenes and amphiboles the fact that calcium is 

 an essential constituent of so many feldspars which, according to Clarke's 

 estimate, compose 60 per cent of the minerals of the original rocks, makes 

 calcium a more abundant element than magnesium. In the meteorites 

 calcium occurs abundantly in the feldspars and pyroxenes, and as calcium 

 sulphide in the mineral oldhamite. 



In 78 shales the calcium oxide amounts to 3.12 per cent of the rock; 6 

 in 624 sandstones to 3.29 per cent, and in 843 limestones to 41.60 per cent. 

 (See p. 938.) These numbers show that the amount of calcium oxide, as 

 compared with the original rocks, is reduced by more than one-third in the 

 shales and about one-third in the sandstones, and is increased more than 



«Clarke, cit, Bull. 78, p. 39; Bull. 168, p. 15. & Clarke, cit., Bull. 168, pp. 16-17. 



