Vol. 65.] THE CAULDRON-SUBSIDENCE OP GLEN COE. 671 



and the southern margins of the Cruachan Granite are, he says, 

 sharply defined against the schists and easy to follow in the 

 field. These two boundaries are shown by his mapping to be 

 steep and even, and they cannot, therefore, be confounded with a 

 laccolitic margin, but they may readily be interpreted as the 

 approximate edge of a subterranean cauldron. The western and 

 north-western boundary, on the other hand, is of a much more 

 complex type. Here we find the very counterpart of the irregular 

 zone of injection which borders much of the Glen-Coe subsidence. 

 Analogy, therefore, suggests that this intricate fringe is com- 

 parable with the fault-intrusion zone of Glen Coe and that it is 

 essentially external to the main mass of the boss with which it is 

 confluent. 



Important evidence in certain other directions supports the 

 hypothesis of a subterranean cauldron in the case of the Cruachan 

 boss. 



In the first place, at a short distance outside the south-western 

 border of the intrusion, Mr. Kynaston bas mapped a remarkable 

 curving fault, approximately conforming for a distance of 10 miles 

 with the rounded boundary of the granite (fig. 1, p. 614). This 

 is certainly not a straight fault subsequently bent by the intrusion 

 of the granite, since the strike of the schists Avhich it traverses 

 shows no corresponding deflection. Its downthrow, though not 

 great, is towards the granite, as is shown by its effect upon the 

 outcrop of a well marked band of lime-silicate hornfels. It is not 

 impossible, therefore, that this fault is a concentric flanking dis- 

 location, tending to enlarge the scope of the subterranean cauldron 

 by downthrow towards its centre. 



The next piece of evidence is probably of more importance. It 

 is found, in fact, that the phenomena of the ' early' fault-intrusions 

 of Glen Coe can be paralleled in the history of the Etive boss. 



In the south-eastern corner of the Cruachan Granite, 

 Mr. Kynaston (op. cit. p. 96) has separated out the older mass of 

 the Beinn a' Bhuiridh intrusion, elongated in form, slightly over 

 4 miles in length, and measuring half a mile across at its 

 maximum breadth (fig. 1, p. 614). Its outline is strongly curved, 

 in conformity with the margin of the main intrusion ; and it is 

 separated from the Highland Schists outside by an almost con- 

 tinuous composite strip, made up of diorite and granite belonging 

 to the later phases of the Cruachan magma. It 



' traverses elevated ridges and deep corries alike ; it goes up one side of a ridge 

 and down the other side in a manner strongly suggesting the behaviour of a 

 great vertical dyke-like mass. The total vertical thickness of this mass exposed 

 is not less than 2500 feet, while its maximum breadth is half a mile.' 



It consists of a complex of andesite, porphyrite and porphyritic 

 gneissose rocks. The whole series shows the effect of contact- 

 alteration by the Cruachan Granite, while the structure of a 

 Q. J. G. S. No. 260. 2 s 



