Vol. 57.] OCCUKRENCE OF CORUIfJUUM AS A CONTACT-MINERAL. 185 



11. JfoTE on the Occurrence of Corundum as a Contact-Mineral 

 nt Pont Paul near Morlaix (Finistere). By A. K. Coomara- 

 Swamy, Esq., B.Sc, P.L.S., P.G.S. (Head January 9th, 

 1901.) 



In the neii^-hbourhood of Morlaix the schists and quartzites of 

 the Lower Devonian are extensively developed. They are much 

 altered, and often injected by intrusive granite. The altered rocks 

 are described by Prof. Barrois ' as quartzites micaces and 

 leptynolithes. The minerals found in the latter include biotite, 

 muscovite, andalusite, sillimanite, quartz, iron-oxide, chlorite, garnet, 

 staurolite, zircon, graphite, and rarely felspar. The quartzites 

 micaces contain large grains of irregular quartz (not clastic), with 

 flakes of dark and white mica, zircon, iron-oxide, graphite, and 

 garnet. 



Among the intrusive masses, that of Pont Paul is of especial 

 interest. The rock is a granite rather poor in quartz ; the minerals 

 include orthoclase, microcline, oligoclase, quartz, hornblende, biotite, 

 apatite, zircon, sphene, and allanite - (the last two not noticed by 

 myself). The granite is much decayed for a depth of many feet. 

 Included fragments of dark micaceous rocks are to be found in it : 

 they are about the size of a man's fist, and not very abundant. 

 Some may be found m situ, and dug out of the crumbling granite, 

 others by searching the floor of the 



shallow quarry. These included Yig. 1. — Isolated crystals of 

 fragments resemble the Devonian corundum (washed in hydro- 



leptynohthes of the aistnct. fluoric acid). See p. 186. 

 They contain abundantly small 

 tabular hexagonal crystals of blue 

 corundum, first noticed by 

 Prof. Barrois in 1887. 



Examination of thin sections of 

 the included fragments shows that 



the minerals certainly present are rc^ • <-• ^.u u i i 



,. ,.^ . •' '■ ^ L^tnations on the basal plane are 



biotite, muscovite, corundum, faintly indicated. From a photo- 



plagioclase,andalusite,pyrite,mag- graph: x about 9.] 



netite, sillimanite, green spinel, 



and zircon. Orthoclase and quartz may possibly occur. 



The corundum is very abundant, forming conspicuous crystals 

 which exhibit hexagonal or rectangular outlines, and contain com- 

 paratively few inclusions of iron-oxide, very much more rarely 

 sillimanite and zircon, also gas-inclusions. The crystals in a thin 

 section are colourless, or have a faint blue tinge, unequally distri- 

 buted. In the latter case there is slight pleochroism from pale blue 

 for the ordinary ray, to very pale greenish-yellow for the extra- 

 ordinary. Optical anomalies were not observed. Cleavage parallel 



^ Bull. See. Geol. France, ser. 3, vol. xiv (1887) p. 890. 

 - Michel Levy & Lacroix, ' Note sur un Gisemeut fran9ais d'Allanite ' Bull. 

 See. Min. France, vol. xi (1888) p. 65. 



Q. J. G. S. No. 226. o 



