506 B. K. N. Wyllie ct- A. Scott— Plufonics of Garabal Hill. 



altering to chlorite. Spliene occurs in pleocliroic yellow crystals as 

 well as in the common granular form. 



The pyroxene is a pale-green diopside with an extinction of 30°-40° 

 and showing generally schiller inclusions. Occasionally aluminous 

 pyroxenes are also found. The amphibole is the common green 

 hornblende witli low extinction and strong pleocliroism from light to 

 dark green. It occurs partly as the margins of pyroxene crystals, or 

 as a paramorpliic replacement of these, and partly as original crystals. 

 It undergoes alteration in two ways, sometimes to aggregates of 

 pale-coloured actinolite, sometimes to a brown hornblende with 

 a complicated pseudo-schiller structure, which will be described later. 



One of the moi'e acid types occurs on the south-east slopes of 

 Garabal Hill, and consists of quartz, felspar, and green hornblende, 

 ■with subordinate biotite, and chlorite. The felspathic minerals 

 greatly preponderate, though the amount of qiiartz is comparatively 

 small. Sphene is common, and is usually altering to ilmenite or 

 leucoxene. A much coarser rock with abundant sphene and apatite, 

 and no quartz or biotite, is found to the east of Loch Garabal. The 

 felspar is nearly all saussuritized, while the original hornblende is 

 replaced by fibrous aggregates of actinolite or tremolite. Occasionally 

 traces of original pyroxene cores suri'ounded by paramorphic horn- 

 blende are found. In places this passes to a quartz-bearing rock 

 •with a fair amount of biotite, while the hornblende assumes a more 

 prismatic habit than usual. This rock is probably some kind of 

 'sehlieren', as it is more acid than the surrounding rocks, and at 

 other localities, which will be described later, obvious schlieren are 

 found, containing amphibole of similar habit. The diorite which 

 forms the eastern margin of the main davainite area somewhat 

 resembles the Loch Garabal rock. It is finer in grain than the latter, 

 but the felspar is partly replaced by saussurite and the hornblende by 

 actinolite. Quartz is absent, while the presence of original pyroxene 

 is indicated by hornblende aggregates containing traces of original 

 diallage or diopside. A few small flakes of biotite, which is probably 

 secondary, are also present. On the ridge north of Garabal Hill 

 a rock occurs which seems to be the most basic of the diorites ; it 

 probably approaches a gabbro in chemical composition. It is 

 a medium to coarse-grained aggregate of plagioclase, hornblende, and 

 pyroxene, with subordinate orthoclase and biotite and no quartz. 

 The felspar is a comparatively unaltered labradorite exhibiting good 

 twinning, while the hornblende is a green mineral more or less 

 completely altered to brown, and the pyroxene is the pale-green 

 diopside common to the diorites. The felspar occurs in coarsely 

 granular aggregates filling up the interstices between the ferro- 

 magnesian minerals. Mineralogically the rock might also be 

 described as a hornblende-gabbro. 



In the neighbourhood of loc. iii (see Map, Fig. 1) numerous veins 

 and clots occur, rich in epidote. Sections were made of specimens 

 from one such vein which showed some considerable variety. The 

 centre of the vein consisted ef epidote, vesuvianite, and spinel, with 

 occasional crystals of hornblende. The epidote occurs as an even- 

 grained mass of subhedral crystals with pleocliroism from yellow to 



