A. K. Coomdrcmcdniy — Geology of Ceylon. 



17 



(these two witli very smooth fracture) ; brown mottled chert, with 

 phlogopite, spinel, and graphite ; green chert, with abundant mica 

 and graphite, and less frequent spinel ; and whitish decomposed 

 chert, with the same accessory minerals in addition to blue apatite. 



Specimens of limestone with identical accessory minerals occur 

 quite near. On its north side, the chert band appears to pass into 

 decomposed limestone which shows green spots suggestive of partial 

 silicification. It was not quite certain that these apparent transition 

 types occurred in situ. There were, however, many specimens which 

 could only with difficulty be definitely named as chert or limestone. 

 A thinner band of brown chert occurs on the path a little below the 

 main band. 



Partially silicifled crystalline limestone, x 22. 

 C, carbonate (dolomite) ; M, mica ; S, spinel ; remainder, opaline silica. 



Five thin sections of these cherts were prepared. One of the 

 green opal (1056) shows merely a green, structureless, homogeneous, 

 isotropic rock. The brown chert (1052) consists of chalcedonic silica, 

 in characteristic spherulitic aggregates ; there is a colourless trans- 

 parent base in which are scattered very numerous tiny ferruginous 

 aggregates which give the brown colour to the whole rock. Certain 

 cracks are filled with characteristic chalcedonic infiltrations. 

 Eouuded spots containing fewer of the ferruginous specks, and 

 appearing rather dark between crossed nicols, call to mind the 

 appearance presented by structureless radiolarian casts ; the presence 

 of radiolaria is, however, quite out of the question. The greenish 

 micaceous chert (1055) consists of opaline and chalcedonic silica in 

 roughly equal proportions, enclosing numerous individuals of well- 

 preserved phlogopite and a flake of graphite. In another, very 

 similar specimen (1054) the mica is much hydrated, and silica has 

 been deposited between the laminae, which are swollen and dis- 

 placed. In the mottled chert (1053) chalcedonic chert is much 

 more abundant than chalcedony, and the accessory minerals include 

 graphite and abundant and characteristic spinel, colourless in the 



DECADE V. — VOL. I. — NO. I. 2 



