CRYSTALLINE ROC KB OF CEYLON. 107 



from the drawing out by tlowing movement of the various parts 

 of a heterogeneous magma, probably during its gradual consoli- 

 dation under conditions of great pressure. 



The observed relations of the acid and basic varieties show that 

 the latter types were amongst the earlier products of the magma, 

 the more acid types as usual crystallizing later and having a more 

 or less intrusive relation towards the more basic. The continuous 

 flowing movement accompanying intrusion and consolidation has 

 had the effect of drawing out together in bands the acid and basic 

 types and preventing the former from transgressing the latter in 

 the form of dykes. 



No very definite nomenclature can be applied to the different 

 varieties of granulite, which shade insensibly into one another, 

 and are sometimes so intimately interbanded as to make it 

 difficult to collect even a hand specimen of uniform composition. 

 The following general types, however, include the more conspi- 

 cuous varieties, beginning with the most acid : — 



Amongst the rocks of the charnockite series are abundant 

 beds oi granular quartz rock of very various thickness possessing a 

 mineral composition which varies in different cases from nearly 

 pure quartz, through types containing small quantities of felspar 

 and garnet and sometimes mica, to others which are at once 

 recognizable as the more acid types of granulite. We are thus led 

 to regard the quartz bands as genetically connected with the other 

 members of the charnockite series, and in fact, as the last and most 

 acid products of the magma. 



A less acid group is that of the leptynites, rocks composed 

 essentially of quartz and felspar (orthoclase microperthite 

 predominating), with or without more or less garnet and biotite 

 and accessory minerals such as graphite, zircon, and apatite. 

 These light-coloured rocks bulk largely amongst the granulites. 

 Th&garnetiferous leptynites dixe especially conspicuous, the bright 

 red garnets in a nearly white rock giving a very handsome 

 appearance. An especially characteristic feature of some 

 leptynites is the great elongation of the quartz grains. 



Many conspiciuous hills {e.g., Bambaragala near Teldeniya, 

 Central Province), with rocky summits weathered into " tors," 

 consist almost entirely of a granular granite-like leptynite 

 composed essentially of quartz and orthoclase microperthite, but 

 often with a small amount of biotite as well. In such oases the 

 rock seems to form a large ovoid mass amongst more basic types. 



Typical charnockite consisting of quartz, felspars, and hyper- 

 sthene has a very different aspect, being of a greenish colour and 

 usually coarser grain, and having a greasy lustre. A considerable 



li 25-03 



