87 G MR JOHN S. FLETT ON 



groundmass in long, narrow prisms of a rich brown colour. In cross-section it shows 

 the usual faces (110, 010), the prism faces being best developed. In longitudinal section 

 it is sometimes ten times as long as broad, and the ends are irregular or forked. 

 Twinning is common (on 100), and the only enclosures are apatite and magnetite. The 

 pleochroism is — c, dark brown ; h, pale brown ; a, clear yellow — absorption ; r>b»a. 

 From the measurement of twinned crystals the angle t '■ c is about 10°. In all the 

 sections the hornblende is quite undecomposed. The felspar lies between the other 

 ingredients, and being evidently the last to crystallise, shows less perfect crystalline 

 form. Much of it is plagioclase, with both albite and pericline twinning, and often 

 zonal. The long, narrow crystals lie scattered iu every direction ; around them is often 

 a considerable amount of simply twinned orthoclase, never with crystalline outlines, 

 and commonest in the areas richest in felspar. The plagioclase it surrounds shows 

 sharp idiomorphism towards the orthoclase, which has been the last mineral to develop. 

 According to the extinction of the twin lamellae, the plagioclase is principally a variety 

 between andesine and labradorite. Both minerals are quite fresh ; they enclose grains 

 of magnetite and hornblende and apatite in long needle-like crystals. 



To prove the presence of orthoclase, several grammes of the rock were crushed and 

 sifted. The powder selected had an average diameter of -^ inch. This was cast into a 

 Thoulet's solution of specific gravity 2*62. The grains which floated were removed and 

 introduced into a Sollas's diffusion column. The indicators used were pure adularia and 

 cordierite of specific gravity 2*59. None of the particles floated so high as the former, 

 and the great majority had a specific gravity of between 2"58 and 2*59. The felspar is 

 hence an anorthoclase. When removed, washed, and examined under the microscope, 

 they had all the characters of orthoclase felspar. The enclosures were apatite (sp. gr., 

 3*16 to 3'22) and possibly needles of hornblende. 



In structure the rock is panidiomorphic, all the constituents, except orthoclase, 

 showing sharp crystalline outlines ; it is noncrystalline, no trace of glassy base being 

 present. The arrangement of the groundmass is mostly perfectly irregular. Around 

 the olivine phenocrysts the later hornblendes are often grouped with their axes parallel 

 to the crystal faces, a rough approach to ocellar structure ; and in places there are pale 

 spots rich in felspar, which may be 3 or 4 mm. in diameter. The long plagioclases 

 converge towards the centre. Between them is orthoclase and a little hornblende in 

 long radiating prisms ; a cavity filled with calcite occupies the centre, and into it the 

 sharp points of the surrounding crystals project, showing that it is really of miarolitic 

 origin. (See PI. I. fig. 3.) 



The dykes in the shore and in the east burn are neither so fresh nor so coarse-grained 

 as the centre of the large dyke described, but they are practically identical with it in 

 composition and in structure. As we approach their edges, all the dykes become finer- 

 grained, but retain the panidiomorphic structure. There is no trace of a glassy selvage. 

 Olivine and augite are the phenocrysts ; hornblende does not seem to have formed in 

 the first generation ; hornblende and plagioclase form the groundmass ; there is little 



