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PROCEEDINGS OP THE GEOLOGICAL SOCIETY. 



[Dec. 5, 



great foci of igneous action. Wherever we find granitic and sy- 

 enitic eruptions, there the gneiss appears in these more coarsely- 

 crystalline and hornblendic forms. Any person who examines the 

 western gneiss carefully will find these varieties prevailing only in 

 places where intrusive granite and syenite rocks abound, as, for 

 instance, near Scourie and Loch Inver, in parts of Lewis and 

 Harris, and on Loch Greinord. And it is these intrusive igneous 

 rocks, or rather the interior masses, of which these veins are the 

 mere external indications, which have expanded, and tilted up the 

 western gneiss, and thus produced that line of fault and compres- 

 sion which I have pointed out in the sections above described. So 

 far, therefore, from furnishing any objection to the theory main- 

 tained in this paper, the fact that the western gneiss has been thus 

 powerfully interlaced, swollen out, and modified, by veins and beds 

 of red granite, syenite, and other hornblende-rocks, by furnishing 

 a veritable and sufficient cause for the fracture and disturbance ob- 

 served along the line of junction, adds one more proof of its truth. 



It is often assumed that the fine-grained gneiss, mica-slate, and 

 clay-slate are younger than the coarse-grained gneiss and horn- 

 blende-rocks ; but on what grounds I have nowhere seen stated. In 

 the Southern Grampians I have shown that the very reverse relation 

 prevails, and that the clay-slates and mica-slates may be seen 

 troughing the central gneiss both on the south-east and north- 

 west. We see this along the great line of fracture intersecting the 

 primary formations, from the Murray Firth to the Linnhe Loch, and 

 still more on the southern margin of the Grampians. This remark- 

 able line of fracture, dividing the Old Red Sandstone from the pri- 

 mary formations, is the exact counterpart of the great line of fracture 

 now shown to exist in the north-west. As in the south, Ave find it 

 bringing up fine-grained gneiss, mica- or talc-slate, and even clay- 

 slate, succeeded further to the east by coarse-grained and hornblendic 

 gneiss. 



Conclusion. 



Before concluding, I must state that, even had it been proved that 

 the mica-slate or fine-grained gneiss of Sutherland truly overlaps 

 the quartzite, and that this overlap is the result of subsequent de- 

 position, the fact would not bear out the conclusions that have been 

 deduced from it, or establish that entire revolution in Scottish geology 

 which has been supposed. Proof would still be wanted that the 

 mica-slate of Loch Emboli and Loch Hope is inferior to the great 

 masses of granitic gneiss in the centre of Sutherland. We might 

 ask for a continuous section through the interminable moors of the 

 Moin, and for evidence that the Kyle of Tongue and the huge syenite 

 domes of Ben Laoghal and Ben Stomino do not break the series and 

 bring up anew the lower and older gneiss. But such continuous 

 sections have never even been attempted, either there or through the 

 wilds of Assynt and Strath Oykill, still less across the mountain- 

 fastnesses of the Dirry Moor and Fannich Forest, so as to assure us 

 that no older underlying gneiss comes up there. Till this is done, 



