224 PROCEEDINGS OF IHE GEOLOGICAL SOCIETY. [Nov. 16, 



and for several miles across the strata, as in other localities to 

 which I shall afterwards call attention, the order of succession is 

 complete and unbroken, whether there be or be not any rock of 

 igneous origin between the upper limestone and the dark micaceous 

 schists and gneissose flags. Yet these last are coloured in all maps 

 as if they were of the same age as the fundamental gneiss, though 

 they are separated from that rock by enormously thick deposits of 

 Silurian and Cambrian age, and have a strike entirely discordant to 

 that of the older gneiss. (See the sketch-map PI. XII. in Vol. xv.) 



My present aim therefore is, in the first instance, to select, within 

 the tract forming the north-western portion of Sutherland, those 

 districts where in breadths of a few miles (across the strike) the 

 sedimentary strata representing the Lower Silurian quartz -rocks and 

 limestones are seen to pass up into, and to be conformably succeeded 

 by, other overlying crystalline rocks, often highly micaceous, some- 

 times quartzose, and occasionally gneissose, without any general 

 break, albeit bands of igneous rock occasionally occur near the lines 

 of junction, as they do indeed at intervals throughout the whole of 

 the ascending series. 



The greater masses of the igneous rocks which either break 

 through the deposits or are associated with certain strata at different 

 and very various horizons will be spoken of hereafter. 



Transverse Section from the west end of Loch Stack and the moun- 

 tains of Ben Stack and Ben Strome, to the east end, or head, of Loch 

 More. — Having described a general section in Assynt from the fun- 

 damental gneiss, through Cambrian and Lower Silurian rocks, to 

 overlying gneissose and micaceous flagstones, I now call attention to 

 natural sections, where, the Cambrian rocks being absent, the lower 

 gneiss is at once unconformably surmounted by Lower Silurian rocks 

 similar to those of Assynt, which, from the lower quartz-rock up- 

 wards through limestones to the overlying quartzose, micaceous, and 

 gneissic flagstones, also form a complete and continuous series. 



1. Quartz-rock ; a portion of that which extends far to the west. 2. Lime- 

 stone. 3. A band of felspar-rock, which in parts has the character of horn- 

 stone-porphyry. 4. Hard dark-grey limestone, similar to that below the igneous 

 rock. 5. Quartz-rock, with gneissic lamina?, particularly near its base. 6. Thin 

 course of limestone, 6 inches only, splitting into gneissic lamina?. 7. The base 

 of the chief mass of overlying " gneiss," which occupies so large a surface on the 

 banks of the Oykel. 



In describing this section, which so clearly proves the transition from the 

 underlying quartz-rock and limestone into the so-called gneiss, Prof. Harkness 

 well observes that the thin band of intercalated porphyry in no way disturbs the 

 parallel arrangement of the sedimentary rocks. It has either, he says, insinuated 

 itself between the layer of limestone, or has been ejected anteriorly to the depo- 

 sition of the upper limestone. "From the lower portion of the quartz-rock," 

 he adds, " to the uppermost gneiss (both inclusive), there is a uniform south- 

 easterly dip of about 45° ; and the whole are seen distinctly passing under the 

 flaggy gneiss. The same mode of association can be traced to Loch Ailsh and 

 the Eiver Cashly, for a distance of many miles, and is well seen at both ends of 

 the Loch, where the limestones and their accompanying strata present themselves 

 in well-developed masses." 



