33s holland : charnockite series. 



Chemical Composition. 



In view of the fact— (1) that there are certain general differences 

 between sedimentary and igneous rocks in composition, and (2) that 

 the bulk analysis of rocks are only altered to a limited degree by 

 metamorphism, chemical analysis of a gneiss, or of any rock meta- 

 morphosed by physical processes, should afford a clue to its origin. 



By the application of these principles to a number of analyses of 

 gneisses, Rosenbusch found that whilst some agree with known igne- 

 ous types, other gneisses show no chemical similarity to rocks of 

 igneous origin. He concluded, therefore, that the first class of gneis- 

 ses, which he calls orihogneisses, are merely deformed igneous 

 masses, whilst the other types {par a gneisses) are composed of altered 

 sediments. 1 



The most general chemical differences between igneous rocks 

 and mechanically formed sediments are due to the removal in solution 

 of the alkalies and alkaline earths during the decomposition of the 

 former. This process is, of course, only partially accomplished when, 

 as is more often the case in cold climates, disintegration exceeds 

 decomposition ; so that a metamorphosed arkose may chemically 

 differ but slightly from a foliated granite. Gneisses so formed would, 

 however, be limited in distribution and thickness, and the chemical 

 evidence thus increases in value with the size of the formation. 



The few chemical analyses which have been made show that 

 the types included in the charnockite series contain the alkaline 

 bases like their mineralogical equivalents amongst ordinary igneous 

 rocks. In the type mass of charnockites at St. Thomas' Mount 

 which contains much microcline, potash exceeds the soda, whilst 

 in the intermediate and basic types the latter alkali is in excess. 

 In other respects also the four types of the charnockite series present 

 a general chemical likeness to many published analyses of granites, 

 diorites, norites and pyroxenites respectively. 



1 "Zur Aufassung der chemischen Natur des Grundgebirges." Tschermak's min. und 

 petr. Mitth., Vol. Xll (1891), p. '49. 



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