Vol. 65.] THE CAULDRON-SUBSIDENCE OP GLEN COE. 653 



Coire Odhar-mh6r (fig. 9, p. 654). — West of Stob Mhic 

 Mhartuin, exposures are poor until Sron a' Choire Odhar-bhig 

 is reached. On the face of this ridge looking down into Coire 

 Odhar-mhor, the two lines of flinty crush met with in Stob Mhic 

 Mhartuin are again well exposed. The southern one is smashed 

 by a later movement following the old line of weakness and 

 shattering even the north-north-east porphyrite dykes. The grey 

 porphyrite, representing the fault-intrusion, occupies, at this point, 

 the whole distance between the two planes of movement. On the 

 north it exhibits a chilled undisturbed edge against a banded flinty 

 crush-rock, which on its other side passes imperceptibly into the 

 strongly sheared margin of the early fault-intrusion. This is, it 

 will be remembered, the type section for illustrating the difference 

 of age of the two fault-intrusions. It is also interesting, as having 

 furnished a specimen of flinty crush-rock containing numerous 

 well-defined margarites and longulites (12934). 



The map shows clearly the steep uptilting of the lavas away 

 from the boundary-fault. It also indicates the position of the 

 small patch of breccia, situated between the two branches of the 

 fault, which has been discussed already on p. 626. A little north 

 of this apparently sedimentary breccia, stands a crag of quartzite 

 which also shows signs of brecciatiou, but of a kiud that one can 

 readily refer to dynamic agencies. The great interest of this crag 

 lies in the fact that if is traversed by several irregular veins of flinty 

 crush-rock, which wind through it like so many intrusions of 

 felsite. These veins (12333 and 13929) consist of typical, banded, 

 black flinty crush-rock, spangled with specks of quartz, just as are 

 the flinty crush-rocks of Stob Mhic Mhartuin. But these veins were 

 not manufactured in situ. There is no shearing of the country- 

 rock parallel to their margins, nor is there any regularity in the 

 course which they follow : in fact, in many cases they extend as 

 tongues, entering culs de sac in the quartzite, and everywhere 

 exhibiting a delicate flow-banding which accurately conforms with 

 the intricacies of the margin. Clearly in this case the flinty 

 crush-rock has been injected as a fluid away from its source of 

 origin. Just as Stob Mhic Mhartuin, then, furnishes the most 

 convincing evidence of the mode of formation of this curious 

 rock, so does the crag in Coire Odhar-mh6r prove conclusively 

 its fluidity. 



One of these veins of flinty crush-rock traverses, not only the 

 quartzite but also a small intrusion of pink felsite, thus indicating 

 the existence of ' early ' felsites in this locality. As a matter of 

 fact there is good evidence for assigning a whole suite of such 

 intrusions to some early epoch in the development of the cauldron- 

 subsidence. They occur in the form of short irregular dykes and 

 are always cut across by the north-north-easterly porphyrites. 

 They are extremely abundant in this neighbourhood (a few only 

 have been picked out on the map), and, what is especially significant, 

 they have precisely the same distribution in the field as the 



