496 PEOF. T. G. BONNEY AND MAJOR-GEN. C. A. M'^MAHON • 



a foot thick, included in the great mass of gabbro near Carrick Luz. 

 This was shown to one of us in 1886 by Mr. Teall. The rock is of a 

 pale pinkish red colour and looks like a f els tone or microgranulite, 

 being much more fine-grained than either the granitic rock of the 

 granulite, or the granite which is intrusive elsewhere in the serpen- 

 tine. Under the microscope it exhibits a microcrystalline structure, 

 the quartz and felspar (rather decomposed) forming a mosaic of 

 rather polygonal grains ; but there are one or two larger grains of 

 felspar with an irregular outline indicative of a porphyritic structure, 

 and ill one or two instances the grains of this mineral are arranged 

 in short " streaks." There is some little iron oxide, a flake or two of 

 colourless mica, and a grain or two of (?) zircon. The evidence as to 

 the relation of this rock to the gabbro is not decisive, but appearances, 

 macroscopic and microscopic, favour the idea of its being an included 

 fragment. 



(3) A fragment of slaty rock of a pale greenish grey colour, some- 

 what splintery in shape, in the same mass of gabbro on the west side 

 of the headland. The line of demarcation between it and the gabbro 

 is sharp, and it is obviously not a concretionary patch. The slaty 

 fragment is perfectly compact *. It has a sp. gr. of 2*90, a hard- 

 ness of 5 to 5'o, and it fuses very readily with intumescence to a 

 dark brown coloured glass which is not magnetic. It is partially 

 soluble in hot hydrochloric acid, and still more so in hot sulphuric 

 acid, the solutions j'ielding lime, magnesia, alumina, and a little 

 iron. The residue was readily soluble in hot hydrofluoric acid. 



A thin slice of the slaty inclusion examined under the microscope 

 is seen to consist of a colourless hornblende, profusely dotted over 

 with granules of sphene. The hornblende, being without colour, does 

 not exhibit any pleochroism. The refraction-index is normal, 

 judged by the relief and the well-marked character of the outlines ; 

 but the double refraction, indicated by the colours in polarized light, 

 is unusuallj'' weak. The mineral is closely packed together in small 

 lath-shaped, irregular club-shaped, and in idiomorphic prisms : here 

 and there it is somewhat platy, or even leafy, in form. A cleavage, 

 running with the length of the prism, is often well developed ; but 

 occasionally the prism is divided by a single transverse cleavage. 

 One well -developed idiomorphic prism gives the typical prismatic 

 cleavages intersecting each other at angles varying from 128° to 

 125°. Extinction, measured from a single cleavage, ranges from 13° 

 to 19°, and averages 16°. Cross-sections exhibit an optic axis in 

 polarized light inclined to the plane of the section, and prisms and 

 sections showing a single cleavage have the major axis of elasticity 

 at an angle of about 74° to the plane of cleavage on the side of the 

 prism. The whole of the groundmass appears to be composed of this 

 lime-magnesia-alumina hornblende, very poor in iron. 



Hornblende, as is well known, when melted under the conditions 

 which obtain in a laboratory, consolidates in the form of augite — 

 never as hornblende; and the existence of augite crystals surrounded 



* Examined by Gen. M Mahon. 



