540 MESSES. HILL & KTNASTON ON KENTALLENITE [Aug. 1 9OO,. 



position. And, moreover, since the relative proportions of the two 

 felspars in kentallenite vary considerably, the composition of the 

 groundmass, or second generation, of the different varieties would 

 range from that of a mica-diorite to a mica-syenite, the intermediate 

 stage being typical of the monzonite-group of Prof. Brogger. These 

 considerations are strongly in favour of the origin of 

 these and similar basic rocks by a process of magmatic 

 differentiation, in which progressive crystallization 

 was the principal determining factor. 



Finally, it is perhaps not unworthy of note that the characteristic 

 behaviour of the orthoclase is common to many of the different types 

 of this class of basic rock and their less basic allies. It occurs 

 frequently in broad patches which enclose the earlier formed 

 minerals, especially the plagioclase,in a well-marked poecilitic manner. 

 This is well seen, as we have alread}^ pointed out, in the kentallen- 

 ites of the Ben-Bhuidhe area, in shonkinite, and in the olivine- 

 monzonite of Smalingen. It is again exemplified in the norites of 

 the Cortlandt Series and in the monzonites (augite -syenites) of 

 Southern Tyrol. 1 We have not noticed it in any of the Argyllshire 

 intrusions of less basic type than kentallenite, but it is exceedingly 

 well shown in a rock recently collected by our colleague Mr. J. 

 Grant- Wilson, forming a marginal facies to the Ben-Nevis granite, 

 and showing close relationships to the Monzoni type. This latter 

 case is of interest, in that the main mass of the Ben-Nevis granite 

 closely resembles that of Ballachulish and Ben Cruachan, so that it 

 would not be unreasonable to expect the occurrence of rocks 

 belonging to the Kentallen or Shonkin type in the Ben-Nevis area. 



Summarizing briefly the more general petrological features of the 

 rocks under consideration, we may say that kentallenite belongs to 

 a peculiar class of basic rocks, of extremely local occurrence, but 

 now becoming more generally recognized, in which orthoclase and 

 an intermediate or acid plagioclase are associated with such basic 

 minerals as olivine, augite, and sometimes also hypersthene, biotite 

 being also generally present; and these rocks include such types as the 

 shonkinite of Montana and the olivine-monzonite of Scandinavia. 



More particularly speaking, kentallenite may be defined 

 as a rock consisting of essential olivine and augite, 

 with orthoclase and plagioclase in varying propor- 

 tions, and biotite. Hypersthene may occasionally occur in 

 addition to augite, and the biotite is of later formation than the 

 plagioclase. With a decrease in the proportion of olivine, the pro- 

 portion of orthoclase rises, frequently exceeding that of the plagio- 

 clase, and exhibits a micropcecilitic structure. Chemically, so far 

 as is yet known, the silica-percentage ranges from 48 to 52. 

 Magnesia may be as high as 15*26 per cent., while the relative 

 proportions of the alkalis vary. We will now deal with the more 

 special relationships of these rocks to the other intrusions with 

 which they are associated in the field. 



1 See Brogger, ' Die Eruption sfolge der triadischen Eruptivgesteine bei 

 Predazzo in Siidtyrol' Kristiania 1895, pp. 56 & 57, figs. 1 & 2. 



