878 MR JOHN S. FLETT ON 



spots are few ; the grain is finer (the hornblendes averaging "03 mm. in breadth) ; and 

 augite, which is not very abundant in the former rock, is practically absent here, even 

 where the material is fresh enough to ensure its identification if present. There is 

 apparently no orthoclase. This rock is a typical camptonite in the sense in which the 

 term is used by some authors. 



The Dyke at Stromness. — Just outside Stromness harbour, on the isthmus which 

 connects the Holms with the Mainland at low water, there is a porphyritic camptonite 

 dyke which differs from these two in some important respects. The phenocrysts are 

 olivine (serpentinised) and augite, very similar to that of Rennibuster. It is of a pale 

 violet colour, sometimes rather greenish, and has a narrow marginal zone of a slightly 

 darker tint. Flakes of hornblende in parallel growth are scattered through the augite. 

 There are no hornblende phenocrysts. Magnetite occurs in rather large areas, fringed 

 with biotite scales (PI. I. fig. 5). 



The groundmass contains augite and hornblende in about equal amount, in crystals 

 of good crystalline form, and not more than four or five times as long as broad. On the 

 surface of the olivine little hornblendes are planted in ocellar fashion. The augite and 

 hornblende are often in parallel growth. Augite forms the centre, hornblende the 

 periphery, or each may form one side of a crystal, and give its outlines to that part it 

 constitutes. Prisms, also, are common, of which the ends are hornblende, while the 

 centre is augite. Together they form one-half of the groundmass. The rest is felspar 

 in long, narrow prisms, often aggregated to form radiate white spots with a central 

 area of calcite. 



North Galton. — Another porphyritic dyke of great interest occurs at North Galton, 

 Sandwick (PI. I. fig. 6). The phenocrysts are olivine (entirely altered into serpentine, 

 magnetite, and calcite) and augite of a brownish-violet tint, like the titaniferous augites 

 of many basalts, and with a marked dispersion of the axes. As usual, it is perfectly idio- 

 morphic, and encloses flakes of brown hornblende in parallel growth. At the margins the 

 groundmass is that typical of the camptonites, consisting of hornblende and augite in 

 about equal proportions, the augite brownish-violet, the hornblende reddish-brown. 

 Both are frequently twinned, and parallel growths are exceedingly common. Here also 

 the augite is internal, central, or lateral ; the hornblende external, peripheral, and 

 sometimes terminal. The augite was evidently the first of the two minerals to finish 

 crystallising. Plagioclase felspar forms the other half of the groundmass in long, narrow 

 prisms, while felspathic spots of radiating crystals are numerous. 



In the centre of the dyke we have quite a different structure (PI. I. fig. 6). The 

 groundmass is much coarser ; it contains felspar, partly in long, narrow polysynthetic 

 crystals, partly in large, broader, simply twinned crystals. It is unusually abundant 

 in this rock, and the only other mineral of the second generation is brown hornblende. 

 Both minerals are partly idiomorphic, the hornblende much less so than usual. Around 

 the augite phenocrysts lath-shaped plagioclases are arranged parallel to the crystal 

 faces, and the interstices between them are filled up by hornblende, which is entirely 



