Vol. 65.] THE CAULDRON-SUBSIDENCE OK GLEN COK. 631 



the development of the flinty crush-rocks (12329). The isolated 

 quartz grains, so characteristic of many of the Glen Coe crush-rocks, 

 have been produced in this manner, and their freedom from strain - 

 shadows, when embedded in a matrix showing perfect flow-structure, 

 furnishes microscopical proof that this matrix transmitted stress as 

 a fluid (12332). This in itself is strong additional evidence in favour 

 of the view that flinty crush-rocks have undergone partial fusion due 

 to mechanical causes. Incipient crystallization has also been noticed 

 in the base of the crush-rocks from the Glen Coe district (11461 & 

 12931), but it is not impossible that the appearances observed may be 

 the result of contact-alteration due to neighbouring igneous masses. 



In most cases the texture of the ultimate base of these crush-rocks 

 is too fine-grained to be resolved, even with the aid of a J^-inch 

 objective : no more can be recognized, in fact, than that much of 

 the material between the larger grains is of the nature of a fine 

 rock-powder. It is interesting to note, however, that many of the 

 flinty crush-rocks of Glen Coe (12332) carry an innumerable swarm 

 of minute black specks distributed evenly enough throughout 

 their ground-mass, and that a similar feature has been described by 

 Sir Thomas Holland as characteristic of the ' trap-shotten ' bands 

 of India. 



Flinty crush-rock along definite fault-planes in the Glen Coe 

 district may attain the thickness of an inch. In connexion with 

 local shear-belts flanking the major dislocations it is generally found 

 as a mere shred or film. In the one observed case of independent 

 intrusion away from a movement-plane, the rock formed a highly 

 irregular sheet, sometimes exceeding an inch in thickness. 



It is only at comparatively few points that flinty crush-rock has 

 been found along the course of the Glen Coe boundary-fault. Good 

 examples have been noticed in both branches of the fault at and 

 near Stob Mhic Mhartuin, and also in the Dalness and Cam Ghleann 

 sections, details of which are given on pp. 650-53 & 660. 



Much more commonly the position of the fault is marked by an 

 ordinary loose fault-breccia, or ' ruttle,' which weathers out as a line 

 of hollow traversing the hill-slopes. This, the normal condition of 

 affairs, is exemplified along the whole line between An t-Sron and 

 Dalness, and also in many localities north of Glen Coe. Shattering 

 of this type is, however, quite unconnected with the production of 

 the original Glen Coe fault, since it affects innumerable north-north- 

 east porphyritedykes which cross the fault without any other sign 

 of disturbance, and are, accordingly, of later date than the period 

 of subsidence. Indeed, where these dykes cross the unshattered 

 exposures of the fault — as, for example, in the Stob Mhic Mhartuin 

 and Cam Ghleann sections — they are themselves quite free from 

 any marks of disturbance, despite the intense shearing and trituration 

 affecting the rocks which they traverse. Reference will be made to 

 another clear indication that the open shattering along the fault-line 

 is a later and trivial phenomenon, when the ' fault-intrusions ' come 



