Yol. 56.] AND OTHER IGNEOUS ROCKS IN ARGYLLSHIRE. 545 



quartz. As we pass from this marginal area into the more central 

 and slightly coarser part of the mass, the rock is seen to diminish 

 gradually in basicity. Thus slightly nearer the centre we have a 

 quartz-diorite or tonalite [8635], see PL XXXI, fig. 2, 

 consisting of green hornblende, biotite, idiomorphic plagioclase, and 

 interstitial orthoclase and quartz ; while the central portion itself 

 consists of a medium-grained biotite-granite, with biotite alone 

 representing the ferromagnesian constituents [7412]. 



Thus we find in this one small intrusion a perfectly clear and 

 gradual transition from a rock rich in pyroxene, and undoubtedly 

 allied to our still more basic types, to an acid biotite-granite. The 

 pyroxene dies out from the margin inward, while hornblende 

 makes its appearance, gradually becoming an important constituent, 

 and the proportion of orthoclase perceptibly rises. Finally, as we 

 approach the centre of the mass, the hornblende disappears, the 

 biotite persists, and the proportion of orthoclase is still further 

 increased ; and we have a biotite-granite, similar to the more acid 

 granites of Ben Bhuidhe and some of the finer-grained varieties of 

 Glen Pyne. The transition from one type to the other is on the 

 whole gradual, but is more rapid near the margin where the 

 pyroxene-bearing zone commences ; that is to say, the passage is 

 rather more gradual between granite and tonalite, than it is between 

 tonalite and augite-diorite. There can be no doubt whatever of the 

 genetic relationships of the different varieties — for here is a mass 

 showing a progressive increase in the proportion of ferromagnesian 

 minerals from centre to margin, accompanied by a decrease of 

 orthoclase and quartz. 



We will not discuss here the particular type of differentiation which 

 is illustrated by this intrusion : what we wish more particularly to 

 point out in this connexion is the relationship which evidently 

 exists, not only, as this mass alone shows, between granites of the 

 Ben-Bhuidhe type and the augite-diorites (containing interstitial 

 orthoclase) of the same area, but also between these granites and our 

 more basic rocks rich in olivine and augite. Thus, we have already 

 pointed out the relationship between the kentallenites of the 

 Brannie and An-Sithein burns and augite-diorites, such as that 

 of the Clachan Hill ; and we have now described a case of an almost 

 precisely similar variety of augite-diorite actually passing by inter- 

 mediate stages into a granite, within the limits of a single small 

 intrusion. We believe then, from the evidence supplied by a study 

 of these small intrusions in the Ben-Bhuidhe area, that there are 

 genetic relationships between the remarkable group 

 of rocks which we have called kentallenites, and 

 biotite-granites and tonalites of the Ben-Bhuidhe 

 and Grlen-Fyne type. 



Let us now see whether in the neighbouring eruptive centres we 

 have any evidence which will confirm the conclusions arrived at in 

 the Ben-Bhuidhe area. 



In the outlying portions of the region which has been invaded by 



