Vol. 56.] CEYLON ROCKS AND GRAPHITE. 609 



straight extinction with regard to the edge P/M (that is, the trace 

 of the clinopinacoidal cleavage), the extinction of the lamellae does 

 not differ from that of the orthoclase more than from 2° to 4°. 

 That is, for the plagioclase-lamellae the extinction on P (0 1, OP) 

 is from 2° to 4°. This fact, correlated with the extinction on 

 M (0 1 0, oo P X) ) indicates that the lamella) probably belong to an 

 'oligoclase-albite' and not a labradorite. Accordingly the plagioclase- 

 spindles are spoken of in this paper as albite-lamellae, though 

 it is very possible that the felspar is not the same in every case. 



Microcline, where it occurs, is found to contain similar albite- 

 spindles, forming microcline-microperthite. The microcline is 

 easily distinguished from orthoclase in the usual way. 



IV. The Graphite : its Mode oe Occurrence and Origin. 



Graphite is Ceylon's most important mineral product. Its export 

 began before 1830, and now amounts to about 18,000 tons yearly. 

 The Ceylon graphite is remarkably pure, and it is often found in 

 large masses free from other minerals : specimens weighing more 

 than 5 cwt. have been thus obtained. An excellent account of the 

 Ceylon trade in graphite has been given by Mr. A. M. Ferguson [12]. 

 F. von Sandberger [14] described the Ceylon graphite mineralogio- 

 ally ; and it has been used among others by W. Luzi ' in his 

 chemical researches on the mineral. 



While graphite is not unfrequently found in the igneous rocks of 

 Ceylon in the form of small scales and strings, it is as yet only 

 where it occurs as a vein-stone that it has been worked commercially. 



Its singular mode of occurrence in branching-veins was first noted 

 by Prof. Walther [18] on the eastern banks of the Kaluganga, about 

 six hours' sail from Kalutara. Here, from the 



' light red laterite exposed in the quarry to a depth of 12 metres a system of 

 much-branched veins of black graphite stands out most conspicuously.' 



The laterite-matrix was free from graphite, which occurred only in 

 the veins. He supposes that fissures originated in the unweatbered 

 gneiss by dislocations, and were filled up by graphite : 



' When at a later period the gneiss decomposed into laterite, the carbon-filled 

 veins remained unaltered and occur in the laterite to-day, a telling proof that 

 this rock was formed in situ and has not been remanie.' 



% The occurrence of graphite at Ragedara as a vein-stone was noted 

 by Prof. Zirkel and Dr. Diersche ; I have also found it in the 

 Baddegama district. 



The vein-graphite shows coarse platy, or stem- or needle-like, 

 forms arranged at right-angles to the edges of the veins. Sometimes 

 other layers of graphite-elements follow, inclined at a slightly 

 different angle, representing probably a second period of deposition. 

 Graphite may occur also in radially arranged groups of crystal-like 



1 Berichte d. Deutsch. Chern. Geselisch. vol. xxiv (1891) p. 4085 & vol. xxv 

 (1892) pp. 214, 1378. 



