442 MR. J. PARKINSON ON AN INTRUSION OF GRANITE [Aug. 1 899, 



often arranged in little groups ; the components of these may be 

 either separated the one from the other, or closely packed together ; 

 not infrequently the brown mica is associated with this hornblende 

 in irregular films and small flakes. A peculiarity of the hornblende, 

 which is here not uncommon, is noteworthy in connexion witli 

 the derivation of this mineral from augite. An irregular area or 

 spot, central or subcentral in position, is seen in the larger flake>', 

 distinguishable from the surrounding hornblende by its lighter 

 colour. In some cases it appears very slightly fibrous ; it may be 

 associated with rust-coloured dust, suggesting a ferruginous ex- 

 cretion, and usually contains minute colourless and irregular grains. 

 The irregular polarization of these augitic remnants, as they pro- 

 bably are, prevents a definite extinction from being observed. 



A colourless mineral, which deserves some notice, is present 

 abundantly in this section of altered diabase, and occurs in scarcely 

 less profusion in that previously described. It is, indeed, a 

 usual and characteristic feature in all such sections of modified 

 rock from the neighbourhood. The crystals have a strongly-marked 

 tendency to occur in parallel groups, often fifteen individuals or so 

 in each, but adjacent groups may lie at a high angle to each other. 

 Not infrequently they are found in little clusters, without any 

 orientation or radial arrangement. The refractive index is fairly 

 high, the double refraction low, but this may be due to their 

 extreme thinness. The latter quality prevents fhe extinction from 

 being recorded with certainty, but I think that it is very probably 

 straight. The crystals appear broken, but not bent, and the 

 larger examples often have a cross-fracture or cleavage ; occasionally 

 segments of the same crystal are separated by other constituents. 

 The mineral lies with indifference in all the other constituents, and 

 often extends from one to another. That it is not produced in the 

 diabase by mere contact may be inferred from the fact that the 

 crystals are absent from a fresh, unaltered diabase-inclusion, lying 

 in the rock of intermediate character which has been described 

 above. Although I have failed to obtain conclusive evidence re- 

 garding the extinction, I infer the mineral to be sillimanite from 

 its general habit. 



A section has also been prepared showing the contact, which is 

 very obscure and irregular, between the altered diabase and a 

 vein of mixed rock, very basic-looking, which contains large rounded 

 orthoclases. It is of interest, since it shows the remains of an 

 augite corroded by quartz. The mineral, which is of a very pale 

 green and rather fibrous, is altering to dark green hornblende. 

 Sundry grains, non-pleochroic or very feebly so, which are found 

 in small groups, for the most part embedded in quartz, are also 

 most probably the remains of augite. These are on the granitic 

 side of the junction. The basic material included in the granite 

 consists of fairly well-formed fiakes of hornblende and mica, and, 

 in addition, of clusters of interlocking grains of the former mineral 

 resembling the glomeruli noticed above. Occasional flakes of mica 

 are mingled with these. The quartz forms, where it occurs, a kind 



