984 A TREATISE ON METAMORPHISM. 



appears that aluminum, the most abundant of metals, is only about one- 

 third as abundant as silicon, and only about one-sixth as abundant as 

 oxygen. The metal takes third place among the elements. Reckoned as 

 an oxide, in 1891 Clarke estimated the aluminum as 15.04, and in 1900 

 as 15.41 per cent. As an oxide it stands second only to silicon. 



Aluminum has its chief source in the silicates, which occur very abun- 

 dantly in the original rocks. The aluminous silicates occur also in the 

 meteorites. By far the most important of the silicates bearing aluminum 

 are the feldspars, which, as Clarke calculates, compose about 60 per cent 

 of the mass of the original rocks. A large part of the remainder of the 

 aluminum is in the pyroxenes and amphiboles, which, as Clarke estimates, 

 compose 18 per cent of the original rocks. A smaller amount of the 

 aluminum is in the micas, which, as Clarke calculates, compose 4 per cent 

 of the original rocks." Very subordinate amounts of alumina occur in other 

 silicates, the more important of which are nephelite, leucite, and sodalite. 

 Alumina occurs also in sufficient amount in corundum, gibbsite, and other 

 oxides to deserve mention. 



In 78 shales the alumina amounts to 15.47 per cent,'' in 624 sandstones 

 to 5.37 per cent, and in 843 limestones to 1.28 per cent. From the analyses 

 given by Clarke it appears that as a result of the denuding and metamor- 

 phic processes alumina accumulates slightly in the shales, as compared with 

 the amount present in the original rocks, and in the sandstones diminishes 

 to about one-third, and in the limestones to a small fraction. According to 

 Clarke's figures, the increase in the amount of alumina in the shales over 

 that in the original rocks is trivial. This increase in the shales is sufficient 

 to account for only a small part of the deficiency in the sandstones and 

 limestones. The equation 



15.47X.65+5.37X. 30+1. 28X. 05=11.7305 



gives us the average of the alumina for the sediments. ' Therefore the 

 analyses give a deficiency of 3.68 per cent of alumina, or nearly one- 

 fourth, as compared with the original rocks. When it is remembered that 

 aluminum is by far the most abundant of the metals, it will be appreciated 

 that this deficiency is enormous. 



a Clarke, cit., Bull. 168, p. 16. ^Clarke, cit., Bull. 168, p. 17. 



