232 PROCEEDINGS OE THE GEOLOGICAL SOCIETY. [NOV. 16, 



schists, I simply view the latter as accidental and lithological varia- 

 tions which have no trne geological bearing. Thus, in the north of 

 Sutherland and in the clear sections to the east of Loch Eriboll, the 

 mere occurrence of more mica in some of the beds than in others 

 affords no reason for the insertion of a great mass of mica-schist, 

 which, by the unconformity of delineation as marked on the maps, 

 is made in one part of the tract to succeed to the quartz-rock, and 

 in another to an intercalated mass of " gneiss." The fact, on the 

 contrary, is, that both the so-called " gneiss " and the mica-schist 

 are part and parcel of the same great band of overlying strata, the 

 varying mineral members of which have a similar dip and strike. 

 In other words, the one of these flagstones is simply a continuation 

 of the other ; and, whether more or less micaceous, felspathic, or 

 quartzose, they all distinctly overlie the purely quartzose strata of 

 the same Lower Silurian series. So little indeed have some of these 

 overlying beds the aspect of the old gneiss, that Prof. Harkness 

 declares that they often reminded him rather of altered flags of 

 Carboniferous age in Ireland. When, however, these same and 

 other overhang beds are followed further eastwards, in their slight 

 inclination, to the environs of Tongue, they here and there assume 

 more gneissic characters than the strata which clearly He beneath 

 them, — a phenomenon which will be presently spoken of, when it 

 will be shown that in the eastern region such strata are more 

 altered and are infinitely more affected by the intrusion of large and 

 extensive masses of igneous rock than in the north-western country. 



Igneous Rocks of Sutherland. — The oldest igneous rock of Suther- 

 land is the bright-red granite which, penetrating the fundamental 

 gneiss in both large and small masses, seems in some places to be 

 almost a constituent of those ancient rocks which compose the 

 oldest or " Laurentian System." 



Resting upon all this Laurentian or fundamental gneiss, the base 

 of the Cambrian rocks of Canisp is characterized by the large -cry- 

 stalled porphyry, first observed by Mr. Peach as clasping round the 

 lower part of that mountain. This peculiar porphyry not having 

 been detected in any overlying Lower Silurian rock, we may consider 

 it for the present to be characteristic of the Cambrian age in the 

 North-western Highlands. 



No igneous rock has yet been observed to be associated with the 

 lower quartz-rock of Assynt ; but in the limestones which succeed 

 a band of trap or syenitic greenstone has been noted at Inch-na- 

 dampff, which dips with and conforms to the strata ; and although 

 there is no doubt of the intrusive character of this rock (seeing that 

 here and there it greatly modifies the character of the limestone), 

 still it in no wise interferes with the regular general succession of 

 the strata. 



Next, an igneous rock of felspathic character, with some varieties, 

 which, though termed porphyries, are rather syenites (rarely true 

 porphyries), breaks through the upper quartz-rocks far above the 

 limestone of Assynt. In the tract to the east of Assynt which is 

 traversed by the road to Oykel Bridge, they spread out into large 



