430 • RECENT WOEK OF THE GEOLOGICAL SURVEY 



thrust-planes or inclined at a higher angle. The basic dykes on 

 the northern slope of Glasven show no perceptible alteration except 

 where they have been traversed by some of the powerful Post-Lower- 

 Silurian disruption-lines ; but they show in a marked degree the 

 foliation produced by the Pre-Cambrian movements. 



The alteration of the Archaean rocks is more pronounced above 

 the horizon of the Ben-More Thrust in Assynt. Along the uncon- 

 formable junction of the gneiss with the Cambrian and Silurian 

 strata, the former has entirely lost its original structure, and has 

 been converted into greenish epidotic schist, the dip of the foliation 

 being E.S.E. In one remarkable case, to be immediately described, 

 where the gneiss is overlain by the Cambrian conglomerate, the 

 schistosity developed in the latter passes downwards into the 

 former, irrespective of the original bedding of the Cambrian beds or 

 the original foliation of the gneiss. The new structures in the 

 gneiss, along the junction-line, have been produced by the more 

 rapid movement of the upper layers of displaced materials, vdthout 

 destroj^ing the geological relation between the two. In other words, 

 there has been a differential movement of the several layers of the 

 thrust-masses as well as of the constituent particles over each other. 

 Along the eastern slope of the Ben-More range, northwards to 

 Glendhu, where powerful thrusts follow each other in rapid succession, 

 bearing forward slices of the old Archaean platform with the quart- 

 zites, the new divisional planes are very prominent. Further, in 

 the case of the dolerite dykes, new foliation-planes have been pro- 

 duced along the disruption lines, which are much more conspicuous 

 than in the displaced dykes above the Glencoul Thrust-plane. 



Advancing eastwards to the belt of sheared gneiss and green 

 schist underlying the Moine Thrust-plane the evidence relating to 

 regional metamorphism is of a most remarkable kind. The gneiss 

 occupying this horizon between Loch More and Strathcarran 

 possesses new divisional planes, the old ones having been almost 

 wholly effaced. The dip of the foliation is E.S.E. , being more 

 or less parallel with that of the thrust-plane. On the foliation- 

 surfaces close parallel lines, like those of slickensides, are met 

 with, indicating the direction of movement, varjdng from 20° to 40^ 

 S. of E. The divisional planes are also coated with a thin film 

 of sericite-mica, while " eyes " of felspar are drawn out in the direction 

 of the movement. The pegmatites, too, have been sheared, so that 

 their foliation-planes coincide with those of the gneiss, the original 

 quartz and felspar appearing now as thin, close, red and white 

 parallel streaks in the direction of the movement lines. Notwith- 

 standing this extreme alteration of the Archaean rocks south of 

 Loch More, the patch of limestone occurring in the heart of the 

 mass is still recognizable as belonging to the two lowest limestone 

 groups of Durness. 



Along this same belt of sheared gneiss and green schist between 

 Assj'nt and Loch EriboU the Archaean gneiss has been rolled out into 

 a finely laminated slate or slaty schist (mylonite), breaking into thin 

 folia like leaves of paper. All the various stages of deformation. 



