Vol.57-] . OF SOUTH-CEXTRAL CEYLON. 203 



forms run parallel with the general foliation, and from the field- 

 evideuce seem clearly to be an integral part of it. A slide cut from 

 a fairly coarse specimen of the granite shows that it is essentially 

 the same rock as that which forms the lighter-coloured parts in the 

 cuttings near Hambukkana (p. 199). Both rocks are characterized 

 by microcline and microperthite, and by the presence of large, very 

 irregular quartz-grains, while the ferromaguesian silicates are repre- 

 sented in both by a few flakes of mica. A thin section taken from a 

 specimen showing the banding of this rock with the biotite-gneiss 

 demonstrates that no line of demarcation can be drawn between the 

 two varieties, that is, that the whole must have solidified at the same 

 time. The darker part, that called biotite-gneiss, consists of an inter- 

 locking mosaic of quartz and felspar, with orthoclase and plagioclase. 

 A little microcline is found, and one or two grains of orthoclase 

 contain microperthite. Biotite with a slight foliation is the sole 

 ferro-magnesian silicate, and possesses the same characters as before. 

 The specimen is almost identical with one from near Rambukkana, 



A little farther north we come to a very slabby, rather fine- 

 grained, and uniform pinkish ' granite,' the planes of parting being 

 determined by mica-flakes. This ' granite ' forms the greater part 

 of the cutting, but the gneiss also a])pears, and specimens were 

 taken showing the junction of the two. A thin section proves a 

 perfect gradation between the two rocks, and both are characterized 

 by the same type of structure. 



The pink rock contains no ferromaguesian silicate at all (except 

 the black mica just referred to), and is distinguished by a quantity 

 of orthoclase with microperthitic intergrowth, though the mineral 

 does occur without it, and a little plagioclase. The rest of this 

 rock consists of quartz in small and very irregular grains, which 

 frequently occur as inclusions in the felspar and vice versa. The 

 other part of the slide (that is, the gneiss) contains orthoclase with- 

 out a microperthitic intergrowth, large and very irregular quartzes, 

 some plagioclase, and no microcline ; in addition, we find a few 

 flakes of mica. In passing from the pinker rock to that just 

 described the first sign of change is the appearance of the larger 

 grains of quartz, and then the gradual disappearance of the micro- 

 perthitic structure. It seems clear that the pinkish ' granite ' is a 

 fine-grained representati-ve of the coarse rock which streaks the 

 biotite-gneiss in the small quarry above described. 



The rocks of Kadugannawa, Hambukkana, and Mahaiyawa are 

 usually well foliated, and commonly banded. The dark bands are 

 characterized by green hornblende in varying quantity, and the 

 association with this of a brown mica. Garnets are found locally. 

 Field-evidence shows that the inter-relationship of the light and 

 dark bauds may be best explained by the streaking together of the 

 component parts of a magma which had undergone difl'erentiation, 

 and that occasionally this process was carried to a rather unusual 

 extent. 



p2 



