A. K. Coomdraswdmy — Intrusive Pyroxenites, Ceylon. 367 



4. There are good exposures of pyroxenites at Kadawat Oya 

 bridge, between Belihul Oya and Halpe (near the 99^ mile-post), 

 Sabaragamuwa. A number of pyroxenic sills and dykes occur, and 

 very heavy hornblendic and pyritic types are characteristic. The 

 dykes vary from mere threads up to \^ yards in width. Some show 

 very clearly penetration of the pyroxenic material along joints 

 (Fig. IV). Several dykes have quite narrow central zones, con- 

 sisting mainly of pyrite ; in one case large individuals of pyrite 

 occurred at intervals along this line. Some of the larger dykes 

 have central portions composed largely of carbonates with abundant 

 accessory minerals (amphibole, phlogopite, spinel). 



A section (1122) from the point marked X in Fig. IV shows 

 a rock consisting almost entirely of pale sea-green diopside with 

 a very little pyrite, apatite, and phlogopite ; the diopside has a very 

 carious structure, the cavities being occupied by semi-ophitic plagio- 

 clase. The whole dyke is about 6" wide. A section (1123) of 

 another dyke, 6"-8" wide, proves it to consist mainly of very 

 pale sea-green diopside, with some strongly pleochroic phlogopite, 

 yellowish-green amphibole, green spinel, pyrite, apatite, and calcite. 

 The diopside tends to granular forms, moulded by the mica and 

 spinel, the latter being apparently almost the last formed mineral, 

 occupying interstices amongst the grains of pyroxene. As the 

 junction with granulite is approached the mica dies out, and some 

 felspar and pink garnet come in. One other dyke, very dark in 

 colour, consisted mainly of hornblende (especially in the central 

 zone), pale bluish green in thin section (1150), and diopside with 

 a trace of plagioclase occupying interstices ; the hornblende has 

 a strong tendency to crystalline form, even when in contact with 

 diopside (a reverse relation is found in the central zone of the dyke 

 shown in Fig. Ill, PI. XX, p. 366, see description above). 



5. ' Thread dykes ' and others slightly wider. These very narrow 

 dykes appear as dark lines crossing foliation on the surface of rock 

 exposures (Fig. V). Under the microscope there appears nothing 

 more than a rapid concentration of hornblende along a zone perhaps 

 \ inch wide (1103). These very narrow dykes are often in more 

 or less parallel groups of two or three (Fig. V) ; sometimes they 

 intersect at acute angles. Occasionally they widen out into dykes 

 an inch or more wide, clearly visible in the field, but with such 

 indefinite margins as to be hardly recognizable in hand-specimens. 



A few additional examples may be briefly referred to. 



6. A sill of grey-green pyroxenic rock 6-8 feet thick is exposed 

 in the river bed below the road, about 10^ miles from Badulla on 

 the Bandarawela road. The rock (1109) is a coarse aggregate of 

 diopside and scapolite with scattered idiomorphic sphenes ; other 

 varieties consist practically entirely of diopside, individuals of 

 which reach ^ inch in width. The rock is very tough. Somewhat 

 similar rocks are found, not quite in situ, on the slope below the 

 69th mile-post on the Badulla Nuwara Eliya road, and some of these 

 are very handsome coarse aggregates of diopside, amphibole, and 

 scapolite, with calcite and idiomorphic sphene (often over i inch 



