Vol. 56.] AND OTHER IGNEOUS ROCKS IN ARGYLLSHIRE. 549 



Between these basic lamprophyres rich in augite and the typical 

 camptonites of the Ben-Bhuidhe area, every gradation may be traced, 

 by an inspection of the numerous sills. Thus in some sills we find 

 that the proportion of augite is comparatively small, while a brownish 

 hornblende of the type characteristic of camptonite makes its appear- 

 ance, and the proportion of felspar rises. A sill occurring about a mile 

 south of Meall-an-Tighearn illustrates this transition. A section 

 [8834] shows a few chloritic pseudomorphs after augite, brown horn- 

 blende in acicular prisms, more or less idiomorphic plagioclase, and a 

 small quantity of interstitial quartz. -The proportion of biotite varies 

 greatly in these sills, but its absence is unusual. With the increase 

 of hornblende the augite disappears, and we find variations of the 

 camptonite-type, which are exceedingly common. An example, which 

 perhaps shows a higher proportion than usual of ferro-magnesian 

 constituents, occurs | mile west of Clachan Hill, close to the mass 

 of augite-diorite. It may be termed a biotite-camptonite, and 

 is rich in both biotite and hornblende [8462], With a decrease in 

 the proportion of these minerals we have more typical camptonites, 

 and hornblende-lamprophyres in which hornblende is conspicuously 

 porphyritic; and these latter, by the advent of porpbyritic plagioclase 

 and an increase in the felspar of the groundmass, pass over into 

 hornblende-porphyrites. 



In the Ben-Cruachan area similar gradations among the lampro- 

 phyre-sills may be observed. The lamprophyres are here more 

 common in the eastern portion of the area, and numerous varieties 

 of the camptonite-type are exceedingly abundant in Glen Orchy. 



As an example of the more basic end of the series, we may 

 cite a well-marked sill occurring about | mile south-east of 

 Dalmally Hotel. It consists of a medium-grained, dark -grey 

 rock. Under the microscope [8464] the rock is seen to resemble 

 the augite-lamprophyres of Ceann Garbh, near Ben Bhuidhe, and 

 the augite-diorite, allied to kentallenite, of Clachan Hill. It is 

 exceedingly rich in pyroxene, while the proportions of biotite and 

 felspar appear to vary in different parts of the sill. The augite is 

 plentiful : it is almost colourless, and frequently shows well-marked 

 idiomorphism. Besides the augite, one notices also fairly numerous 

 brownish phenocrysts resembling bastite-like pseudomorphs after 

 enstatite. There is no hornblende. The biotite occurs mostly in 

 irregularly-shaped elongated flakes, which are seen to be moulded 

 on the plagioclase, and therefore of later formation. The plagioclase 

 occurs occasionally as idiomorphic porphyritic individuals, which are 

 usually rather decomposed, and as more or less lath-shaped individuals 

 in the groundmass. Interstitially there is a small amount of quartz. 

 A very small quantity of orthoclase may be present, but owing to 

 the decomposition of the felspars it is difficult to identify the mineral 

 with certainty. Occasionally one may see an interstitial substance 

 bordering some of the plagioclase-crystals, and showing a minute 

 more or less fibrous structure, suggestive of micropegmatite. 



As in the Ben-Bhuidhe area, so here we may trace the connexion 

 between these basic sills and the camptonites through intermediate 



