544 MESSES. HILL & KYNASTON ON KENTALLENITE [Aug. IC)OO r 



also occurs in characteristic ragged plates, and is of late forma- 

 tion. We perceive, however, that hornblende has appeared, plagio- 

 clase is plentiful, and the proportion of orthoclase-felspar has 

 dwindled to an exceedingly small amount. The rock may he classed 

 as an augite-diorite, showing affinities on the one hand tokental- 

 lenite, in the occasional presence of olivine, the character of the 

 augite, the behaviour of the biotite, and the presence of a small 

 amount of orthoclase ; and on the other hand to diorites of the 

 tonalite-type, in the presence of plagioclase, hornblende, and a 

 certain amount of interstitial quartz. 



A fairly similar rock is found forming part of a small intrusion 

 | mile south of Beinn Chas, on the ridge between the head of Brannie 

 Burn and Glen Fyne. It appears to form the more marginal 

 portion of the intrusion, and to pass, towards the more central 

 portion, through a diorite of the tonalite-type into biotite-granite. 

 The intrusion itself is about J mile long and J mile broad, and is 

 situated rather over a mile from the boundary of the main mass of 

 the Glen-Fyne granite, though a marginal area of schist extensively 

 veined by granite approaches it to within 3 mile. A specimen from 

 near the margin of this mass is a dark-grey fine-grained rock of 

 dioritic appearance. Under the microscope [7413], see PI. XXXI, 

 fig. 1, augite is seen to be plentiful in more or less idiomorphic 

 crystals and rounded grains. Biotite appears as small ragged flakes, 

 and behaves in the same way as the biotite of the Brannie and An- 

 Sithein rocks. The rest of the rock consists of small lath-shaped 

 plagioclases, interstitial orthoclase, and a small amount of quartz. 

 There is also some accessory magnetite, and some secondary green 

 hornblende and chlorite in association with the pyroxene. There is 

 no olivine. This rock is evidently a variety of augite-diorite, 

 resembling that already described from the neighbourhood of 

 Clachan Hill, and bearing relationships, borne out by the high 

 proportion of pyroxene, the behaviour of the biotite, and the 

 presence of interstitial orthoclase, to our basic rocks of the Brannie 

 and An-Sithein burns. The entire absence of olivine separates it 

 from our kentallenite-group, and brings it a step nearer to the 

 diorites of this district. At the same time, the presence of ortho- 

 clase allies such rocks as these with the syenites, and especially with 

 the typical raonzonites (augite- syenites) of the Tyrol. 



Another specimen from the margin of the same mass [8637] 

 rather tends to show that a basic modification, exactly resembling 

 the rock just described, does not necessarily constitute an uniform 

 zone along the entire edge of the mass, but rather that the margin 

 may be less basic in some parts than in others, the different varieties 

 shading imperceptibly one into the other. We see in this specimen 

 that the proportion of augite has become relatively small, that there 

 is still less idiomorphism in the crystals, and that they are often 

 partly replaced by green fibrous hornblende. Augite, in fact, has 

 sunk to the level of an accessory constituent, while the main mass of 

 the rock consists of numerous small idiomorphic plagioclases, small 

 biotite-flakes, green hornblende, and interstitial orthoclase and 



