606 ME. A. K. COOMAEA-SWAMY ON [Aug. I9OO, 



inclusions may occur in definite planes. The grains are not attacked 

 by drops of hydrofluoric acid placed on the slide. The light-coloured 

 succeeding portion of the rock consists chiefly of quartz and 

 plagioclase. 1 



(14) Quartz-Calcite Micropegmatite. 



A graphic intergrowth of quartz and calcite has been described 

 above (p. 605 & PI. XXXIII, fig. 1). I believe that such an inter- 

 growth has not been previously observed. Graphic intergrowths, 

 probably resulting from the simultaneous intergrowth of two or more 

 minerals forming an eutectic mixture, 2 are well known to occur 

 in rocks which have consolidated from igneous fusion. It is hardly 

 probable, however, that in the present case the quartz and calcite 

 have consolidated from igneous fusion. It is easier to suppose that 

 we are dealing with a vein-product, the result of a kind of solfataric 

 action, rather than with a true magma-rock. Quartz and calcite 

 are common vein-products, and often occur together in an ordinary 

 way. It seems to me possible that conditions analogous to those 

 found in consolidating igneous magmas might be produced in con- 

 nexion with the formation of vein-products. In this way a 

 micrographic intergrowth of quartz and calcite might be produced. 



(15) On Centric Structures in Pyroxene- Granulites. 



Intergrowths of pyroxene and plagioclase from Pidurutalagalla, 

 Bagedara, etc. have been described above (pp. 593, 594 & PI. XXXIII, 

 figs. 3 & 4). Similar intergrowths, on a larger scale, of pyroxene, 

 oligoclase, and quartz, and of pyroxene and oligoclase, have been 

 recorded by Lacroix [17] from Salem and Ceylon. He describes 

 from Salem a micropegmatite of amphibole and orthoclase forming 

 a corona 1 cm. thick, surrounding garnets of the size of a man's fist. 



The radiating pyroxene-plagioclase intergrowths of the Piduruta- 

 lagalla rocks may be considered first. Here the 'straits' of felspar, 

 separating magnetite and garnet which would otherwise fit together, 

 have also to be explained. The following is a suggested theory for 

 this case : — Magnetite and garnet were formed early, the garnet 

 often enclosing the magnetite. Pyroxene, and then felspar followed ; 

 in parts of the rock not containing garnets they were able to crystal- 

 lize without interfering one with the other. Some change in the 

 physical conditions occurring at this stage caused the corrosion of 

 the garnets, and perhaps partly also of the pyroxene, by the viscous 

 felspathic magma. Thus channels were eaten out between the 

 garnet and maguetite, and to some extent between the garnet 



1 I hope to be able to obtain more material, and isolate the undetermined 

 mineral. This has not yet been possible, owing to its fine grain and the small 

 amount of material available. 



2 Fr. Becke, 'Die Gneiss- Formation desnieder-oesterreichischenWalclviertels' 

 Tscherm. Min. u. Petr. Mitth. vol. iv(1882) p. 406 ; Teall, ' British Petrography' 

 1888, p. 401 ; Bonney, ' On a Contact-structure in the Syenite of Brad gate 

 Forest' Quart. Journ. Geol. Soc. vol. xlvii (1891) p. 101. 



