280 ME. W. R. WATT 0~S THE GEOLOGY [June I914, 



exception is the occurrence of the platy variet} r of sillimanite in 

 the mica -garnet-rock found in the railway-cutting below Boghead 

 -of Kinnoir. Finer examples have been described in the aureole of 

 metamorphism of the Ross of Mull granite. 1 



The cordierite of these rocks calls for little remark. It is 

 granular, seldom twinned, and may show an abundance of inclusions 

 with, in most cases, the characteristic halo. 



The garnet is the ordinary ' common garnet ' — a mixture of the 

 lime, magnesia, and iron molecules, with the last predominating 

 greatly. It is often full of quartz- and felspar-inclusions, and is 

 the typical spongy garnet of a contact-rock. Occasionally, crystal- 

 faces are rather well developed : one example from the slopes above 

 Cobairdy showed the faces ^(110) and n(211). 



Although the felspars possess considerable range in composition 

 — from oligoclase to labradorite — the majority are andesine 

 and acid labradorite. Orthoclase and quartz are not infrequently 

 intergrown to form ' myrmekite,' and the quartz may show fine 

 hairs — in all probability rutile, as in the quartz of the Netherly 

 diorite. 2 Staurolite is also present in these rocks. 



The rocks of Cumrie Plantation are interesting, in that they 

 show signs of the planes of foliation in the numerous stages of 

 absorption that occur among the xenoliths. Where a true hornfels 

 has been formed, these can occasionally be detected by the develop- 

 ment of bands of minerals — garnet-rich and cordierite-rich bands 

 alternating. 



A different development is observed in the fine-grained rocks of a 

 greenish hue found near Ward. Their colour is due to the presence 

 of a pale-green hornblende, accompanied by cordierite, garnet, 

 quartz, muscovite and biotite, and felspar. A further stage is met 

 with on the slopes of Whitehill, Cobairdy, where the green horn- 

 blende and the garnet are better developed, the faces of the garnet 

 being well defined. 



In the cutting at Rothiemay Station are to be found two very 

 different types. Here occurs between the two main masses of the 

 intrusion a contact-altered rock, containing both cordierite and 

 hypersthene. The cordierite is uncommon, but not so the hyper- 

 sthene, which, like the associated pyroxene (diopside), can be traced 

 from a beautifully pleochroic variety to an indefinite aggregate. 

 In part, at least, the presence of this mineral may be due to the 

 impregnation of the crystalline rock by molten material, which, 

 however, has not sufficed to remove all the flakes of biotite. 



Very different is the pale rotten material to be found on the 

 north side of the cutting. Its white or pink coloration makes it 

 easily distinguishable from the dark igneous rock. The foliation is 

 well marked, and on fractured surfaces the presence of much biotite 

 can be seen to impart to the rock its colour. In section, both fine- 



1 T. O. Bosworth, Q.J. G. S. vol. lxvi (1910) pp. 376-96. 



2 W. Mackie, Trans. Geol. Soc. Edin. vol. viii (1900-1905) p. 85 ; and 

 J. S. Flett, 'Geology of Lower Strathspey' Mem. Geol. Surv. Scot. 1902, 

 Explan. Sheet 85, p. 39. 



