540 B. K. N. Wyllie & A. Scott— Phdonics of Garahal Hill. 



is reached and then increases slightly, while the biotite curve, though 

 more irregular, as is to be expected, is very similar. The hornblende 

 and felspar curves tend to become horizontal at the ' coarse ' end 

 where a nearly uniform rock is reached. Since these rocks show no 

 normative quartz, the latter has obviously been formed from silica 

 which has undergone hydrolysis at high temperatures. The increase 

 in the water content as the coarse rocks are reached favours this. 



The chemical composition, on the whole, d'^es not show any great 

 variations. We have analysed a specimen of the diorite close to the 

 tonalite (VI) and one of the coarse hornblendite (VII). 



VI. Diorite near junction with tonalite, Garabal Burn. 



VII. Hornblendite, Garabal Burn. 



Vila. Approximate composition of brown hornblende of hornblendite (VII). 



VIII. Diorite (very rich in biotite), Ben Damhain, anal. J. H. Player.' 



IX. Granitite (tonalite), Ben Damhain, anal. J. H. Player. - 



Though the silica is more or less constant, the hornblendite is 

 somewhat more basic ; that is, raagnt'sia and lime increase at the 

 expense of the alkalis, while the alumina decreases. These differences, 

 however, are not sufficient to account for the mineralogical differences, 

 as the' norm ' of the hornblendite shows a considerable amount of felspar. 

 Hence we must conclude that the amphibole contains a considerable 

 amount of alkali and lime, since the pyroxene which occasionally 

 forms the core of the hornblende crystals is diopside. The amount 

 of water has undoubtedly been an important factor in the production 

 of the hornblende, though the relatively greater amount of magnesia 

 must also have had an influence. 



Origin of the Browk Hornblende. 

 The case is more difficult when we come to consider the alteration 

 of the hornblende. There is no doubt that the mineral was originally- 

 green, and that the change originated from the centres of the crystals, 

 unless in the case of those with diopside cores where a band of green 



^ Teall & Dakyns, loo. cit., p. 115. " Ibid., p. 115. 



