IX THE NOKTH-WEST HIGHLANDS OE SCOTLAND. 431 



from tlie crushed Archsean gneiss on the one hand, to the lami- 

 nated slate on the other, can be clearly traced. The original 

 constituents of the gneiss have been comminuted, but here and 

 there broken fragments of the felspars occur, which are invariably 

 drawn out in the direction of movement. The colour of the slaty 

 schists has been determined by the nature of the materials out of 

 which they have been made. Where the Archsean gneiss contained 

 much cpidote, the slates or " crush-rocks " are light green ; where it 

 contained much hornblende, they are dark green ; where pegmatites 

 or granitoid gneiss have been the chief materials employed, the 

 resultant slates are red or pink. These finely laminated schists or 

 slates show beautiful examples of fluxion-structure; and their 

 foliation-surfaces display closely set lines or '- striping," indicating 

 the direction of movement of the particles over each other, the 

 general trend of the latter being E.S.E. Associated with these 

 slates are certain belts of " frilled " dark-green schists, of precisely 

 the same character as those so well exposed on the coast-section 

 east of Whitten Head, Loch Eriboll. A detailed study of the 

 remarkable structure presented by these "frilled" schists points 

 to the conclusion that they have been formed by Post-Lower- 

 Silurian movements mainly out of dark hornblendic gneiss, the 

 folia having been piled on each other by minute major and minor 

 thrusts. 



Occasionally, along this belt of sheared gneiss and schist, there 

 are lenticular masses of the original Archaean rocks, which only 

 show partial deformation, and, in addition to these, strips and wedges 

 of Silurian and Cambrian strata which have been completely con- 

 verted into schists. 



2. MetamorpMsm of Cambrian Strata. 



It is interesting to note that no Cambrian strata occur among 

 the displaced masses brought forward by the Gleucoul Thrust in 

 Sutherlandshire. They do not appear till we reach the horizon of 

 the materials lying above the Ben-More Thrust-plane. The various 

 changes produced by these movements in the Cambrian conglo- 

 merates, sandstones, and shales are strikingly exemplified on Ben 

 More, on the north side of the Oykel valley, and on Sgonnan More. 



Beginning with the basal conglomerate, or " Button-stone," we 

 find that it has undergone extraordinary changes, both where it 

 underlies the gneiss in the Oykel valley and where it overlies that 

 rock in Corrie Mhadaidh. In its unaltered form, throughout the 

 undisturbed Cambrian areas, this characteristic band of conglo- 

 merate is composed of more or less well-rounded pebbles of quartz- 

 rock, gneiss, pegmatite, diorite, &c., imbedded in a loose, gritty 

 matrix. But where it has been subjected to mechanical movement, 

 the softer pebbles of gneiss and the fragments of the basic Archaean 

 dykes have been crushed, flattened, and elongated in the direction 

 of movement. Indeed, in some cases, they have been drawn out 

 to such an extent as to form thin lenticular bands of micaceous or 



Q. J. G. S. No. 175. 2 G 



