Vol. 56.] AND OTHER IGNEOUS ROCKS IN ARGYLLSHIRE. 555 



whether diffusion or some other cause, the parent-magma evidently 

 became differentiated into basic and acid portions, the former, 

 judging from the relative positions of the resulting extrusions, being- 

 marginal with regard to the latter. Such differentiation may be 

 said to be of a ' complementary ' character, the extremes of the 

 resulting series representing ' complementary ' rocks, whose average 

 composition would represent approximately that of the original 

 undifferentiated magma. 



Among the intrusions of the dyke or sill- phase we would suggest that 

 the more acid and more basic series thus stand as ' complementary' 

 groups, with transitional types. Prof . Brogger has especially drawn 

 attention to such a relationship among the camptonite- and bostonite- 

 intrusions associated with the olivine-gabbro-diabases of Gran. 



It is probable that the original magma which underwent differ- 

 entiation in our Ben-Bhuidhe area was one of more or less 

 intermediate character, approximating in composition to tonalite, 

 and that a process of complementary differentiation in such a magma 

 produced the augite-diorites and kentallenites on the one hand, 

 and the hornblende- and biotite - granites on the other. The 

 constant association of a generally similar assemblage of rocks in 

 each Argyllshire area is strongly in favour of the view of their 

 origin in each case by differentiation in a common magma. And, 

 moreover, the grouping of the basic rocks in each area about the 

 more acid, as about a common centre, supports the idea of a similar 

 type of differentiation having taken place, along parallel lines, in 

 the respective reservoirs which constituted the source of supply 

 for each eruptive centre. 



VI. Summary. 



We may now briefly summarize the main conclusions at which 

 we have arrived in the foregoing pages. Under the term kental- 

 lenite we have described a peculiar group of basic rocks, which 

 may be denned as consisting essentially of olivine and augite, 

 with smaller amounts of orthoclase, plagioclase, and biotite, while 

 apatite and magnetite are accessory. The most striking feature of 

 the rocks is the peculiar association of alkali-felspar with olivine and 

 augite. The group is related to the shonkinite of Square Butte and 

 Yogo Peak in Montana, and to the olivine-monzonites (Brogger) of 

 Scandinavia. 



Kentallenites are distributed over a large portion of Argyllshire, 

 and appear to be associated with four more or less distinct centres 

 of eruptive activity, each of which is characterized by important 

 intrusions of various granites and diorites, and, in most cases, also 

 by a profusion of dykes and sills of lamprophyres, porphyrites, etc. 

 The kentallenite-group represents the most basic intrusion of each 

 of the areas where it occurs, and it is found in the more out- 

 lying or marginal portions of each of those areas respectively. 



We have pointed out, chiefly from evidence collected in the Ben- 

 Bhuidhe area, that close relationships exist between these kentallen- 

 ites and certain augite-diorites occurring in the same district, and 



