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



IY. The Belation between the Kentallenites and the 

 Lampeophyees. 



We have already referred to the occurrence in the Ben Bhuidhe 

 area of numerous lamprophyres, which are found mostly in the form 

 of narrow sills intrusive in the older schistose rocks. These sills 

 are especially numerous on the Glen Shira side of Ben Bhuidhe, but 

 become far less common as we go westward from Glen Shira. They 

 are thus of more common occurrence in the tract characterized by 

 the kentallenite and augite-diorite intrusions. These rocks are by 

 no means all of one type, but show considerable variation in com- 

 position, ranging from basic augite-lamprophyres through various 

 varieties of camptonite to a hornblende-lamprophyre approaching 

 hornblende-porphyrite. Biotite also, as well as hornblende, fre- 

 quently plays a prominent part in these rocks, and thus biotite- 

 camptonites and various micaceous lamprophyres may also be met 

 with. 



A microscopic study of a large number of these rocks has con- 

 vinced us that there are close relationships between the more basic 

 augite-bearing varieties of the group and the larger intrusive masses 

 of kentallenite and augite-diorite. Let us take one or two examples 

 of some of the more basic varieties of these sills. Two small 

 lamprophyre-intrusions of this type occur on the slopes of Ceann 

 Garbh, about 2 miles north-east of Ben Bhuidhe. These sills are 

 similar in appearance, and consist of a medium-grained dark-grey 

 rock, in which augite and some plagioclase- crystals may be seen to 

 occur porphyritically. Under the microscope [7416 & 7417] a 

 section is seen to consist of fairly numerous and relatively large 

 phenocrvsts of augite, and a few phenocrysts of plagioclase, in a 

 groundmass which is mainly felspathic. The augite is idiomorphic, 

 colourless, and frequently shows a well-marked zonal structure. 



The groundmass consists mainly of small lath-shaped plagioclases, 

 small augite-grains, irregular flakes of biotite, and chloritic altera- 

 tion -products. There is a very small quantity of interstitial quartz, 

 and magnetite i§ accessory. Although there is no original horn- 

 blende in these rocks, yet there is a striking family resemblance 

 between them and the variety of augite-diorite already described, 

 occurring on Clachan Hill, and also to the more basic marginal por- 

 tion of the dioritic intrusion of Beinn Chas. There can be very little 

 doubt that they represent the dyke-phase of the same magma, or of 

 a similarly differentiated portion of it, as that which produced these 

 more basic dioritic types ; and we have already pointed out the close 

 relationships that exist between these latter intrusions and the 

 kentallenites of the Brannie and An-Sithein burns. Although the 

 peculiar and essential characters of kentallenite can scarcely be said 

 to be represented in these sills, yet their intimate association with 

 the more basic intrusions of the area leaves little uncertainty as to 

 the existence of relationships between them. No doubt kentallenite 

 represents a more specialized product of the original common 

 magma. 



