640 MESSES. CLOTTGH, MAT/PE, AND BAILEY OX [Nov. I 909,. 



culminates in flinty crush-rock ; on the other hand, it has deformed 

 the phenocrysts, especially the ferromagnesian elements, which it 

 has drawn out into thin parallel films consisting of aggregates of 

 biotite-crystals. The difference between the streaking of the 

 phenocrysts in this instance, and the intimate crushing of the 

 early fault-intrusion on the northern margin of the cauldron, is very 

 marked, and points to an important difference in the conditions 

 under which the shearing stresses acted in the two cases. It is 

 probable that the streaky margin of the Allt Coire an Easain diorite 

 was sheared before the main mass (of the same intrusion) had risen 

 along the fault, and that it was caught by the movement when the 

 matrix was still pasty. If this supposition is correct, the shearing 

 of the diorite in itself scarcely suggests an early date for the 

 intrusion. On the south, however, evidence is forthcoming, since 

 in this direction the dark diorite is clearly cut across by the pink 

 granitite, which, as has been shown, merges insensibly northwards 

 into the normal grey tonalite of the fault-intrusion proper. 



(7) The dykes, and their relation to the cauldron- 

 subsidence. — Prom the days of MacCulloch the multitude of 

 dykes in Glen Coe has always been a cause for remark. They 

 are so abundant that, in some places, they actually bulk as largely 

 as the country-rock, and consequently, in order not to obscure the 

 other geological features of the district, they have been omitted 

 from the general map. A comprehensive view of their distribution 

 may be obtained by an inspection of fig. 1 (p. 614), but in localities 

 such as Glen Coe, where they cluster thickly, scarcely more than 

 one-tenth of their actual number is represented on the map. 



The vast majority of these dykes are of later date than the fault- 

 intrusion, and traverse the schists, the lavas, and the Eannoch and 

 Cruachan granites alike, always exhibiting clear evidence of chilling 

 at their margins. Their prevalent trend is north-north-east and 

 south-south-west, and it remains unaffected by the boundary-fault: 

 except that very occasionally some of them turn along it for a 

 space, following the fault just as they would any other old line of 

 weakness. Some are broken, it is true, by the shattering, which 

 often characterizes the fault-line ; but, as has been explained above, 

 this shattering is later than the actual subsidence. They likewise 

 traverse the lavas upturned at the margin of the cauldron, without 

 themselves being tilted from the vertical. Occasionally, indeed, a 

 slight hade is discernible : as for instance, near Alltchaorunn in 

 Glen Etive ; but, throughout the district generally, they maintain 

 a remarkably uniform verticality, witnessed by the straight courses 

 which they follow across mountain and glen. 



By far the greater number of these dykes are porphyrite& 

 distinguished by phenocrysts of plagioclase, and one or more of the 

 ferromagnesian minerals (hornblende, biotite, augite or hypersthene). 

 The ground-mass is fine-grained, and consists of felspar, with a 

 second generation in many cases of the ferromagnesian minerals, 



