16 A. K. Coonicirasmiuii/ — Geoiof/i/ of Cei/lon. 



as the pressure of other duties permits. I woukl like to add that 

 my work in the Berwyn Hills was assisted by a grant from the 

 Government Grant Committee of the Eoyal Society. 



Appendix. 



Confirmation of the determination of the felspars in the kerato- 

 j)hyre of Moel-y-Golfa was obtained by uncovering a portion of 

 one of the slides and immersing the thoroughly cleaned edge of the 

 section in ethylene bromide {/u = 1-5355, by the Fuess Refractometer, 

 Model II). The felspar showed 7 a little above /< of the liquid, 

 a distinctly below. The extinction angles on sections perpendicular 

 to 010 of twins on the albite and carlsbad laws also confirmed albite, 

 the measurements on four sections being: — [20 : 22] [18 : 21] ; 

 [17 : 16] [17 : 21] ; [01 : 91] [13 : 16] ; [5 : 6] [3 : 3]. The 

 determination of the felspar of the Berwyn kei'atophyi-e was also 

 confirmed by refractive index measurements, and extinction angles 

 on cleavage chips as well as on symmetrical sections twinned on the 

 albite and carlsbad laws. 



Brief descriptions of Becke's bright line method of determining 

 small difiierences of refractive index, and of Michel Levy's method 

 of determining the plagioclase felspars by the extinction angles on 

 sections perpendicular to 010 of crystals twinned on both albite and 

 carlsbad laws, will be found in the Appendix to Iddings' Translation 

 of Eosenbusch's " Microscopical Physiography of the Rock-making 

 Minerals," 4th ed., New York, 1900. 



V. — Contributions to the Geology of Ceylon : 



11. Silicification of Crystalline Limestones. 



By A. K. CooMAUAswAMY, B.Sc, F.L.S., F.G.S., Director of the Mineral Survey 



of Ceylon. 



THE occurrence of small quantities of chert and opal, usually in 

 or near exposures of crystalline limestone, but very often in 

 fragments or boulders not quite in situ, is not unusual in Ceylon. 

 For some time the origin of these siliceous rocks remained obscure ; 

 observations made within the present year (1903), however, enable 

 me to give a more detailed account of their mode of occurrence. 

 I have had the advantage of my colleague Mr. James Parsons' 

 company in examining many of the exposures, and have been able 

 to discuss with him the problems raised. 



A band of chert occurs in situ on the path descending from the 

 ambalam just J mile W.N.W. of Uduwela trigonometrical station 

 (about 3 miles south-east of Kandy), and about 4 to 5 yards below 

 the fourth of the six bands of limestone which are crossed in 

 descending the hill.^ The width of the band of chert is about 5 feet ; 

 it includes a number of varieties, all with good conchoidal fracture. 

 These are : homogeneous green opal ; homogeneous brown chert 



1 The locality can be identified on the map, Q.J.G.S., vol. Iviii (1902), pi. xiii, but 

 the position of the bauds of limestone is not correctly indicated there. 



