J. G. Ooodchild — Granite Junction in Mull. 449 



Some important problems relating to both physics and chemistry 

 are involved in this matter. These I could not even attempt to 

 solve. But some little additional field evidence gained vphile 

 studying the behaviour of several granite masses in Cornwall, 

 Cumberland, and Westmoreland, in some parts of southern Scotland, 

 and especially in the Eoss of Mull, seems to me to indicate the 

 direction in which further research, systematically carried out, may 

 lead eventually to a correct understanding of the facts. 



Around the border of the Eoss of Mull, chiefly on its eastern 

 and south-eastern margin, the granite contains unusually large 

 quantities of included masses of rock, chiefly quartzites, grey- 

 wackes, and mica schists, identical in character with that of the 

 Highland Metamorphic Series, which the granite here penetrates. 

 In some of the larger granite quarries, as for example, those at 

 Camas Tuadh on the west side of Loch Lathaich, and others again 

 near Ardalanish, these included blocks occur in such profusion as to 

 impart to the rock the appearance of a gigantic breccia, in which the 

 granite itself plays but the role of matrix. In many instances the 

 blocks are as much as twenty or thirty feet in length, and are but 

 little rounded, nor are they much altered beyond the stage of 

 metamorphism observable in these rocks at points far removed from 

 the granite. The parent masses do not appear to have undergone 

 much shearing prior to their engulphment in the granite, so that the 

 lines of stratification, the false bedding, and the joints of the original 

 quartzite are distinctly traceable through the included blocks in 

 their present position. The inclusions appear to have been detached 

 from the parent block by means of joint planes, and, once so 

 detached, to have quietly floated away into the molten rock, without 

 any further change specially noteworthy. 



I had occasion, a year or two ago, to follow the boundary of the 

 granite for some miles, beginning at its clear junction with the 

 quartzites at Ard an Daraich, and following it from that place 

 southward. The task of laying down this boundary upon the map 

 was by no means as simple as it is in the majority of cases. Many 

 granites come against their enclosing rock with a perfectly well- 

 defined junction, often narrow enough to be covered with a knife 

 edge. But the Eoss of Mull granite behaves difi^erently. In some 

 respects, indeed, it may be said to shade off into the metamorphic 

 schists around. It is not intended by this that the mica schists 

 or the quartzites showed an increasing percentage of crystalline 

 felspar in proportion to their nearness to the granite. The micro- 

 scopic structure, as well as the field evidence, showed that this case 

 forms no exception to the now-generally recognized rule in this 

 matter. The rocks are granulitized, as might have been expected, 

 but no transition can be observed from the one mineral type into the 

 other. The gradation referred to is effected by the steady increase 

 in the proportion of the masses included within the granite, as this 

 rock is traced outward towards its peripheral zone. Beyond that 

 zone, where some few masses of schist and quartzite still remain 

 more or less attached to their parent rocks, the number of granite 



DECADE III. — VOL. IX. — NO. X. 29 



