390 RECENT "WOEK OF THE GEOLOGICAL SUEYET 



mainly in the form of dykes, which may be grouped in the following 

 order : — 



a. Basalt-rocks, comprising dolerites, basalts. 



h. Peridotites and palaeo-picrites. 



c. Microcline-miea rocks. 



d. Granites. 



(a) One of the most striking features of the Archaean rocks in the 

 west of Sutherland is the extraordinary series of basic dykes, 

 composed mainly of members of the first group (Basalt-rocks), 

 throughout the tract extending from Lochinver to Loch Laxford. 

 Between Lochinver and Kylesku their general travel is "W.N.W. and 

 E.S.E ; between Kylesku and Loch Laxford the direction is more 

 northerly, and in some instances it is nearly E. and W. Erequently 

 they occur in groups, upwards of fifteen dykes being met with in 

 the course of a mile. So persistent are they, that many of them 

 have been traced for a distance of ten or twelve miles from the 

 west coast of Sutherland, across the Archaean area, till they are 

 buried underneath the Cambrian sandstones and Silurian quartzites. 

 Sometimes they send forth branches or veins, which maintain 

 separate courses for a considerable distance, and ultimately re- 

 unite with the parent dykes. Sometimes basalt-veins traverse 

 dykes of dolerite, the peculiar lithological characters of each 

 being preserved at the points of intersection. In all cases they 

 possess the various zones characteristic of these igneous intrusions : 

 the outer parts are more fine-grained than the centre and along the 

 edges patches of tachylyte are occasionally met with. Only a very 

 few of the dykes preserve the original prisms running at right 

 angles to the walls; but their absence is satisfactorily accounted for 

 by the extraordinary amount of deformation which they have 

 undergone owing to the later Pre-Cambrian movements. Attention 

 will be directed presently to the evidence in proof of the meta- 

 morphosis of many of these dolerites into diorites and hornblende- 

 schists resulting from these later movements. But notwithstanding 

 these facts, it must be admitted that many of the phenomena 

 presented by this grand series of Archaean dolerite-dykes are but 

 the counterpart of what may be seen in the splendid development 

 of basalt-dykes of Tertiary age connected with the volcanic plateaux 

 of the Inner Hebrides. 



(6) The trend of the peridotite- and palaeopicrite-dykes, which are 

 best displayed in the Canisp deer-forest and westwards by Brack- 

 loch, Torbreck, and Eiecairn near Lochinver, is more nearly E. 

 and W. than that of the dolerites. As they traverse the various 

 dolerite-dykes in their path, there can be no doubt that they were 

 erupted after the basalt-rocks ; and the evidence is also conclusive 

 that their date of intrusion was prior to the later Pre-Cambrian 

 movements. Weathering into a dark brown earth, they usually 

 form long narrow clefts or hollows, thus giving rise to conspicuous 

 features in the scenery of the Archaean area. 



(c) On the south side of Loch Glendhu, a dyke consisting mainly of 

 felspar and mica appears in the coast-section trending IN'.E. and 



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