1002 A TREATISE ON METAMORPfflSM. 



rocks. Thus it is that orthoclase and microcline are the chief feldspar 

 minerals which develop by the deep-seated metamorphism of the pelites — 

 rocks in which decomposition has been carried further than in the other 

 mechanical sediments. To a less extent orthoclase is likely to be the domi- 

 nant feldspar in the other sediments. However, since complete decompo- 

 sition for the mechanical sediments is very rare, the potassium for much of 

 the orthoclase and microcline which develop by metamorphism is derived 

 from the undecomposed original minerals. 



BARIUM, STRONTIUM, CHROMIUM, NICKEL, LITHIUM, FLUORINE, BROMINE. 



In Clarke's estimate of 1900 of the lithosphere barium is put down as 

 composing 0.05 per cent, strontium as 0.02 per cent, chromium, nickel, 

 lithium, and fluorine as 0.01 per cent each and bromine as less than 

 0.01 per cent. The quantities of these elements are so small and so little 

 is known about the transformations through which they go that no attempt 

 is here made to consider the influence of metamorphism upon their 

 distribution. 



GENERAL STATEMENTS. 



At the beginning of this chapter it is stated that the attempt to apply 

 quantitative methods to the redistribution of the chemical elements is made 

 more with the idea of laying out problems to be solved than with the belief 

 that the calculations approach accuracy. However, when all of the equations 

 which compare the average composition of the sediments with the original 

 rocks are taken into account the impression is gained that the calculated 

 relative proportions of the three classes of sediments discussed — that is, the 

 shales (including all silicate rocks), the sandstones, and the limestones — can 

 not be far from the truth. The percentages of each of the elements in the 

 sediments and in the original rocks are matters of quantitative determina- 

 tion, by imperfect methods, it is true, and so far as absolute masses are 

 concerned the estimates given for the sediments are but roughly approxi- 

 mate, but the relative proportions stated are probably nearer the truth. 

 Other proportions of the sediments than those used might be selected which 

 would answer in the equations for a single element, and possibly some other 

 proportions than those used might be selected which would accord better 

 with the facts; but it seems certain that such proportions could not differ 



