534 MESSES. HILL & KYNAST0K ON E!ENTALLENITE [Aug. I900 T 



plates in the coarser varieties, and shows a characteristic lustre- 

 mottling. 



As regards the microscopic characters of the type-rock, we cannot 

 do better than quote Mr. TealPs description of a specimen from 

 Kentallen l (see PI. XXIX, fig. 1) : 



' The rock [7053] 2 is composed of olivine, angite, biotite, plagioclase, ortho- 

 clase, magnetite, and apatite. The olivine is fresh, colourless, and is traversed 

 by the usual anastomosing veins of magnetite. It is also, as a rule, crowded 

 with extremely minute and often rod-like inclusions, which give it a cloudy 

 aspect when viewed with low powers. The sections of the individuals are 

 never bounded by straight lines meeting in sharp angles, but traces of idio- 

 morphism are not uncommon ; in other words, the mineral occurs as grains or 

 as crystals with more or less rounded angles. It is present as inclusions both 

 in augite and biotite, and the felspars are often moulded upon it. 



' The augite occurs in grains, crystals, and patches, which vary in size — the 

 largest often measuring several millimetres in diameter. The common crystalline 

 forms may frequently be recognized, and the angles are often fairly sharp. 

 The colour in thin slices is pale green, and sections which appear homogeneous 

 in ordinary light often show a beautiful zonal structure under crossed nicols. 



' Biotite occurs in small ragged patches, which, in spite of their apparent 

 isolation, often show uniform orientation over large areas, The mineral is of 

 a rich brown colour, approximately uniaxial, and strongly pleochroic. It is not 

 only moulded on the augite and olivine, but also occasionally on the felspars, 

 and was therefore one of the last minerals to form. 



' The felspars, together with the small quantity of biotite, make up the inter- 

 stitial matter in which the olivines and augitesare embedded. Both plagioclase 

 and orthoclase are present in approximately equal quantities, and the former is- 

 markedly idiomorphic with respect to the latter. The plagioclase sometimes 

 shows a zonal structure, and probably ranges in composition from labradorite to 

 oligoclase, the acid type predominating. It may be readily distinguished from 

 the orthoclase by its higher refractive index and by the albite-twinning. The 

 orthoclase occurs as interstitial matter (mesostasis). Individual patches some- 

 times show twinning on the Carlsbad plan. The accessory minerals are apatite 

 and magnetite, the latter occurring almost exclusively in the veins which traverse 

 the olivine. All the minerals are remarkably fresh.' 



Passing to the area farther south-east, we find that the above 

 description will apply equally well to sections prepared from speci- 

 mens of the intrusions near Loch Avich [8287] [8568] and in Glen 

 Orchy [8614] [8670] ; see PI. XXIX, fig. 2, & PI. XXX, fig. 1. 

 The Glen Orchy rock, however, appears to be rather richer in 

 olivine and augite than that of Kentallen. The felspathic material 

 is finer-grained, and, together with the biotite, plays the part of 

 groundmass. This is especially well seen in the more marginal 

 portion of the mass, where there is a sharp distinction between a 

 groundmass of biotite and felspar and relatively large and con- 

 spicuous augites and olivines. A slide [8676] from another portion 

 of this mass shows numerous phenocrysts of olivine and augite in a 

 groundmass of biotite and felspar. The olivine is remarkably fresh, 

 and is often seen to be bounded by crystal-faces : it occasionally 

 occurs as inclusions in the augite. The augite is fresh, almost 

 colourless, shows a beautifully defined crystal-form and zonal 



1 Ann. Eep. Geol. Surv. 1896 [1897] p. 22. 



- The numbers in brackets throughout this paper indicate rock-sections in 

 the Geological Suney collection at Jermyn Street, London. 



