652 MESSES. CLOUGH, HATJEE, AND BAILEY ON [Nov. 1909, 



difference between the moved and the unmoved portions of the rock 

 are quite striking on weathered surfaces. The normal rock 1 yields 

 big blocks evenly bounded by joint-planes, and smooth except for a 

 conspicuous pitting due to the weathering out of the phenocrysts. 

 In an early stage, movement manifests itself by the production of 

 a closely interlacing meshwork of cracks, weathering white on the 

 grey face of the rock. Further movement causes obvious displace- 

 ment along these cracks, and the breaking-down of the phenocrysts 

 leads to the disappearance of the characteristic pitting ; the rock no 

 longer weathers in flat-faced blocks, but with a rounded, uneven 

 surface, recalling somewhat in appearance the grey lichen-covered 

 trunk of an old beech. Along special bands of shear, streaks 

 of flinty crush-rock have frequently been induced, which show up 

 black on a fractured face (12933). 



The occurrence of these isolated flinty crush-bands, evidently 

 produced in situ, is of importance as dispelling the idea that the 

 formation of this type of crush-rock has any essential connexion 

 with igneous intrusion. The independent dynamic origin of flinty 

 crush-rocks has, of course, been clearly recognized in all districts 

 where rocks of this type have been studied ; but it is extremely 

 satisfactory to find clear local evidence bearing upon the point. 



Beyond the outcrop of the early fault-intrusion, we pass on to 

 schists which attest that the line of movement, recognized along 

 the southern boundary of the intrusion, is one of great importance. 

 Here, on the northern flank of Stob Mhic Mhartuin, the strike is 

 south-south-west, and the strata are practically vertical. They 

 consist of quartzo-micaceous granulitic flagstones, which certainly 

 must be placed in the Moine Group of the CentralHighland Schists. 



For some distance these schists, or gneisses as they are more 

 properly termed, show signs of movement, incipient and occasionally 

 complete brecciation, and also of local reddening. These effects do 

 not extend for more than about a third of a mile from the belt of 

 maximum disturbance, and evidently are connected directly with the 

 movements of subsidence which have affected the Glen Coe area. 



Lastly, it may be mentioned that the normal north-north-east 

 porphyrite dykes of the district traverse Stob Mhic Mhartuin in 

 great number. They cross all the zones of disturbance without 

 themselves showing any signs of trouble, and they exhibit chilled 

 margins at their contacts with the two fault-intrusions which they 

 traverse without let or hindrance. 



The Stob Mhic Mhartuin section has been dealt with fully, because 

 it presents an epitome of many of the more important features 

 of the Glen Coe boundary-fault. It is fortunate that in this readily 

 accessible section there should be such clear evidence of two definite 

 stages in the Glen Coe subsidence. 



1 ' The Stob Mhic Mhartuin section does not give so good an idea of the- 

 unmoved rock as others lying to the west. 



