18 A. K. Coomdraswdmy — Geology of Ceylon. 



thin section but pink in tbe band specimen. In none of these slides 

 are any remains of carbonates to be found. 



Some specimens collected from blocks resting on crystalline 

 limestone, but not quite in situ, on Upper Rajawela estate (about 

 11 miles from Kandy on the Teldeniya road), about a third of a mile 

 E.S.E. of Rajawela trigonometrical station, were also sliced. Of 

 these one (1074) evidently consisted of partially silicified limestone; 

 the section showed disintegrated and corroded crystals of dolomite 

 embedded in an isotropic siliceous matrix, in which an abundance of 

 hydrated phlogopite and a few grains of spinel ai'e-also found. 



The silica has penetrated along the cleavage cracks of the 

 carbonates, with every appearance of corrosion. The dolomite 

 individuals are thus broken up into irregular fragments, often more 

 or less rhombohedral, and these graduate into the smallest specks 

 which remain scattered in the siliceous base, sometimes indicating 

 by their disposition the rough outline of the original carbonate. 



Another specimen (1073) from this locality consisted entirely of 

 brown chert and resembled No. 1053. 



The amount of chert present in any locality is always small, and 

 quite insignificant in comparison with the total amount of crystalline 

 limestone present ; nor can the occurrences of chert be followed for 

 any distance. They are also met with in other j^arts of Ceylon, 

 e.g. in the Uva Province, although their connection with crystalline 

 limestone is not always traceable ; but there is no direct evidence of 

 their occurrence as a replacement of any other rock. 



From the foregoing observations I conclude that these opaline 

 cherts result from the alteration of crystalline limestone, the car- 

 bonates being dissolved and replaced by opaline or chalcedonic silica, 

 or a combination of the two. Very possibly the pure siliceous rocks, 

 free from accessory minerals, do not so directly replace the limestone, 

 but are siliceous deposits similar to the chalcedony deposited in cracks 

 in the other cherts, which must already have had time to harden and 

 develop cracks, previous to the introduction of a further supply of 

 silica. The silicification is probably the result of the presence of 

 heated waters containing silica in solution introduced after the con- 

 solidation of the crystalline limestone in its present form. In other 

 words we have here ametasomatic transformation. A number of hot 

 springs are known to occur in Ceylon, e.g., at Badulla, Alupota, 

 Bubule, and Bibile, in the Uva Province ^ ; and near Koggala, Magam 

 Pattu, in the Southern Province.'* The occurrence of these springs 

 lends support to the probability of such alterations having taken 

 place. 



It is of interest to notice the bearing of these observations on the 

 origin of cherts in general ^ ; we are here dealing with cherts which 

 are certainly of inorganic origin. The mica, spinel, and graphite met 

 with in the chert are proof that the original rock was a crystalline 



1 Uva Manual, by H. White, Colombo, 1903, p. 82. 

 * Ceylon Admiuistratiou Reports, 1902, Survey Department, p. B. 30. 

 3 For a discussion of this question, see C. A. Raisin, Proc. Geol. Assoc, xviii 

 (1903), pp. 71-82. 



