1000 A TREATISE ON METAMORPHISM. 



per cent. This gives potassium eighth place in abundance among the 

 elements. Reckoned as an oxide the amount in the original rocks, accord- 

 ing to Clarke's estimates of 1891 and 1900, is 2.90 and 2.80 per cent, 

 respectively." As an oxide, potassium thus has seventh place. 



Potassium is an abundant constituent of a number of the silicates of 

 the original rocks. The more important of these are the feldspars and the 

 leucites. Since the feldspars probably compose more than half of all the 

 original rocks (see p. 937), the chief original sources of the potassium are 

 the feldspars. Of the feldspars the content of potassium is so great in 

 orthoclase and microcline as to give them the distinctive name potash 

 feldspars, thus discriminating them from the acid end of the plagioclase 

 series in which soda is the dominant alkali. 



Considering the secondary rocks, the amount of potassa in 78 shales is 

 3.25. per cent, 6 in 624 sandstones 1.24 per cent, and in 843 limestones 0.46 

 per cent. It thus appears that as compared with the original rocks, the 

 amount of potassa in the shales is increased by about one-fifth, in the sand- 

 stones is reduced to less than one-half, and in the limestones to less than 

 one-fifth. Multiplying the percentage of potassa in each class of sediments 

 by the volume of those sediments, and taking their sum, we have the 

 average amount of potassa for the sediments considered, thus : 



3.25 X .65 + 1.24 X .30 + .46 X .05 = 2.5075. 



Since the amount of potassa in the original rocks as given by Clarke in his 

 latest estimate is 2.80 per cent, we have a deficiency of potassa in the 

 sediments of 0.2925 per cent, or 1,974,375,000,000,000 metric tons. 



According to Dittmar's estimates the actual amount of potassium in the 

 ocean is 512,000,000,000,000 metric tons, which, reckoned as potassa, is 

 616,600,000,000,000 metric tons, or 31.23 per cent, i. e. about one-third of 

 the total deficiency. Potassium compounds are also important accessories 

 in salt deposits. For instance, analyses show that the potassium composes 

 from 1.1 to 7.4 per cent of the total solids of Great Salt Lake" and 1.478 

 per cent of the total solids in the Dead Sea. d 



As is well known, above beds of rock salt there are not infrequently 

 overlying beds rich in potassium and magnesium. This class of salts is well 



"Clarke, cit., Bull. 78, p. 39; Bull. 168, pp. 14-15. <'Mon. U. S. Geol. Survey, vol. 1, pp. 253. 



'> Clarke, cit., Bull. 168, p. 17. ''Encyclopedia Brittanica, 9th ed., 1877. 



