REDISTRIBUTION OF MAGNESIUM. 993 



most intimately associated and most closely allied. Reckoned as an oxide, 

 Clarke's estimate of 1891 is 4.49 per cent, and of 1900 is 4.36 per cent. 

 This gives magnesia fifth place among the oxides. Magnesium occurs as a 

 constituent of the original igneous rocks in a number of minerals. Of 

 these, the pyroxenes, amphiboles, micas, and olivines are the more 

 important. It is only rarely present in any considerable projiortion in the 

 feldspars, the most abundant group of minerals. Thus, while occurring- in 

 more minerals than calcium, it is not so abundant as calcium, because of 

 the great role of the feldspars." In the secondary rocks magnesia composes 

 2.45 per cent of 78 shales, 0.85 per cent of 624 sandstones, and 6.20 per 

 cent of 843 limestones. It thus appears that as compared with the original 

 rocks the amount of magnesia in the shales is reduced to about three-fifths, 

 in the sandstones to about one-fifth, and in the limestones is increased by 

 more than one-third. The sum of the products of the percentage of each 

 class of sediments by their proportional amounts gives the average 

 percentage of magnesia in the sediments, thus: 



2. 45X.65+.85X.30+6.20X. 05=2.1575. 



Subtracting this 2.1575 from 4.36, the amount in the original rocks, we have 

 a deficiency of 2.2025 per cent. The total amount of sediments multiplied 

 by this latter percentage gives 14,866,875,000,000,000 metric tons. 



The question now arises as to the explanation of this apparent vast 

 deficiency of magnesia in the sedimentary rocks. The question is not easily 

 answered. Possibly some part of the calculated deficiency is only apparent. 

 Probably the composite analyses of 345 limestones, taken at random, in 

 which the magnesia is 7.90 (see p. 938) is nearer the average for the lime- 

 stones than this amount averaged with the quantity of magnesia, 4.49 in 

 the building stones (see p. 938), since it is well known that the magnesian 

 limestones are likely to be porous and brecciated, and therefore not so 

 suitable for building purposes. If the average amount of magnesia in the 

 limestone were taken as 7.90 instead of 6.20, as given in the above 

 equation, Ave have: 



7.90X.05-6.20X. 05=.085. 



But 0.085 per cent is only about one twenty-sixth of the calculated 

 deficiency, and therefore we must look somewhere else for the explanation 



a Clarke, cit, Bull. 16S, p. 16. 

 MON XLVII — 04 63 



