A. K. Coomdraswduiy — Intrusive Pyroxenites, Ceylon. 363 



V. CONTKIBUTIONS TO THE GeOLOGY OF CeYLON : 



4. InTEUSIVE PyEOXENITES, MiCA-PYEOXENITES, and M1OA-ROCK& 



IN THE Chaenookite Seeibs oe Geanulites in Ceylon. 



By Ananda K. Coomaraswamy, B.Sc, F.L.S., F.G.S., Director of the 

 Mineralogical Survey of Ceylon. 



(PLATE XX, IN TEXT.) 



THE object of the present paper is to provide a short account of 

 a group of rocks to which the general name of pyroxenite may 

 be given, inasmuch as they are essentially non-felspathic plutonic 

 rocks, free from olivine, and of igneous origin, and with a nearly 

 colourless monoclinic pyroxene as the chief or only constituent of 

 many of the most conspicuous varieties.^ The exposures are widely 

 distributed in the Central, Uva, and Sabaragamuwa provinces, and 

 probably throughout Ceylon, but are individually of very small 

 extent. The rocks are composed of the minerals diopside, amphibole^ 

 phlogopite (or biotite), scapolite, pyrite, sphene, and sometimes 

 felspar, calcite, and spinel ; the dominant mineral is the pale green 

 or grey granular diopside, colourless in thin sections : the rocks 

 are often entirely, or almost entirely, composed of this mineral ; 

 next in importance are green amphibole (dark in hand-specimens, 

 but very pale in thin sections), and a pale golden-brown phlogopite, 

 which sometimes forms a large proportion of the rock, and in 

 extreme cases the whole bulk of sills or dykes several feet in 

 thickness. 



The rocks occur in sills and dykes of all sizes, from a fraction of 

 an inch to six feet or more in thickness, and their intrusive character 

 is generally clear. Very similar or identical rocks occur also as 

 nodular aggregates in crystalline limestones, and also near and 

 along junctions of granulite and crystalline limestone^; but for the 

 sake of brevity and clearness, only those occurrences which are less- 

 particularly associated with the crystalline limestones are described 

 in the present paper. The pyroxenites are clearly intrusive in the 

 charnockites or granulites ; although often occurring in beds or sills 

 following the foliation planes of the granulites, they occur equally 

 or more often as typical dykes crossing these foliation planes, and 

 sometimes occupying joints ; the figures illustrate examples of both 

 modes of occurrence, which, moreover, are frequently associated. 



This occupation of joints shows that the pyroxenic material must 

 have been introduced after the granulites had sufficiently con- 

 solidated to allow of the formation of joints. This fact, together 

 with another, viz., the occurrence of what seems to be a very 

 ordinary mica pyroxene dyke cutting the zircon granite (itself 

 belonging to the Balangoda group,^ and forming an intrusive sill in 



1 For a discussion of the nomenclature of similar rocks, see C. H. Gordon " On 

 the Pyroxenites of the Grenville Series in Ottawa County, Canada," Journal of 

 Geology, vol. xii. No. 4, 1904, pp. 324, 325. 



* For descriptions see A. K. Coomaraswamy, "The Crystalline Limestones of 

 Ceylon," Quart. Journ. Geol. Soc, 1902, vol. Iviii, pp. 399-422. 



3 A. K. Coomaraswamy, Geol. Mag., No. 482 (1904), pp. 418-422. 



