546 MESSES. HILL & KYNASTON ON KENTALLENITE [Aug. I9OO,, 



the huge granite-mass of Ben Cruachan, we find several intrusions 

 of a well-marked variety of augite-diorite situated at no great 

 distance from the margin of the granite. A mass of this diorite is 

 well exposed on Ben Lurachan, between Glen Strae and Glenkinglass. 

 One or two smaller masses occur in the same neighbourhood, and 

 the rock is again well seen on Meall-an-Laoigh, between the head 

 of Glen Strae and Glen Orchy. In their mode of intrusion these 

 diorite-masses resemble kentallenite, that is to say, they behave 

 rather as small bosses and broad lenticular sills. In colour the 

 rock is dark grey, the augite being very conspicuous, and generally 

 marked off sharply from the more felspathic portion, so as to give 

 the rock a characteristic spotted appearance. A bronze-brown 

 biotite is also easily distinguished by the unaided eye. Specimens 

 from Meall-an-Laoigh [8622], Ben Lurachan [7760], and Meall 

 Copagach [7761], in the same neighbourhood, were examined under 

 the microscope. 



The augite occurs in relatively large and perfectly idiomorphic 

 crystals, and closely resembles the augite of the kentallenite of 

 Glen Orchy. The margin of the pyroxene is frequently seen to 

 be replaced by a border of green hornblende. Hornblende is also 

 seen to occur as an original constituent, and to be of a variety 

 similar to that observed in the granite and diorite of Ben Cruachan, 

 etc. The rest of the rock consists of biotite, plagioclase, ortho- 

 clase, and quartz. The biotite occurs in ragged patches and groups 

 of detached Hakes in optical continuity, a feature already noticed as 

 characteristic of kentallenite. Similarly, it is of later formation 

 than the plagioclase, which is markedly idiomorphic. The orthoclase 

 and quartz are interstitial. Apatite and iron-ores occur as acces- 

 sories. The orthoclase is well seen in the rock of Ben Lurachan 

 and Meall Copagach ; it is only present, however, in very small 

 proportion in the Meall-an-Laoigh rock. 



There seems to be strong evidence, therefore, of the relationship 

 between these rocks aud the kentallenite of Glen Orchy. The 

 augite and biotite are of the same variety and behaviour, and 

 orthoclase occurs interstitially. In fact, in the Meall-an-Laoigh 

 rock, if we could replace the hornblende by olivine, we should have 

 a kentallenite of the Glen-Orchy variety ; while the rocks of Ben 

 Lurachan and Meall Copagach approach more nearly the type of 

 Kentallen in their higher proportion of orthoclase-felspar. Again,, 

 it can hardly be doubted that these diorites are closely associated 

 with the larger granitic intrusions of Ben Cruachan, finding as we 

 do within the Ben-Cruachan mass an actual transition from augite- 

 diorite to hornblende-granite. 1 Thus, in this area again we find a 

 dioritic rock, occurring in well-individualized intrusions, showing 

 affinities to the kentallenites of the same area, while a similar diorite 

 is seen to occur as part of the Ben-Cruachan mass. 



In the Loch-Avich and Kilmelfort are a several small granitic 

 intrusions have been mapped by our colleague Mr. E. G. Symes, and 

 1 Summ. Progr. Geol. Surv. for 1897 [1898] p. 86. 



