1858.] MURCHISON ^NOETHEEN HIGHLANDS, ETC. 391 



The great physical fact, however, in respect to the series of 

 crystalline rocks in Sutherland, is, that their lower members contain 

 those limestones in which organic remains have been found, and that 

 large masses of chloritic, micaceous, and gneissic flagstones, clay- 

 slates, and schists are so superposed, that little doubt can remain 

 that they are also of Silurian age. Allowing for many undulations 

 and repetitions, I see no reason for believing that those overlying 

 crystalline masses are of much greater thickness than the Lower 

 Silurian rocks of Wales and the adjacent English counties. 



Having no evidence of the existence of other fossil remains 

 which should indicate the presence of Tipper Silurian rocks, and 

 seeing how poorly such strata are developed even in that part of 

 the south of Scotland (Ayrshire) where the Lower Silurians are so rich 

 in fossils, my inference is that the younger members of the Silurian 

 system have never been deposited in the far north. There, a great 

 upheaval of the Lower Silurian, or the quartzo-gneissose series in 

 question, separates it sharply from the Old Eed Sandstone, which 

 overlaps it in entire unconformity, — this great break accounting for 

 the nonexistence of any representatives of the Upper Silurian rocks. 



It is unnecessary that I should continue to combat, as I did at the 

 last Glasgow Meeting of the British Association, the theoretical idea 

 that the varying and alternating mineral layers of the rocks I have 

 been describing are lines of cleavage or lamination as distinguished 

 from true stratification ; since that view can, I presume, no longer 

 have any supporters. In considering these rocks as regularly stra- 

 tified deposits of different mineral matters, I simply adopted the 

 opinion of all my eminent precursors since the days when Hall and 

 Playfair classed them as metamorphosed sediments. Not only have 

 we examples of coarse grits and sandstones throughout them, but 

 even when we follow them to the south we find regular beds of 

 pebbles intercalated in such chloritic and micaceous schists. In 

 short, they present all the signs by which truly stratified aqueous 

 deposits are defined, not only in their successive mineral layers, but 

 also in having wavy bands of schist interpolated among sandy beds, 

 and the whole being afifected by symmetrical joints. 



The presence of organic remains was alone wanting to complete 

 the natural inference which I suggested in 1854, that many of the 

 stratified crystalline rocks of the Highlands are simply the meta- 

 morphosed equivalents of the Lower Silurian rocks of the south of 

 Scotland. 



Crystalline Rocks of the Shetland Isles. — My voyage from point to 

 point of the Shetland Islands was much too rapid to enable me to 

 connect all their difierent rock-masses with those of the mainland. 

 I may, however, suggest that some of the northernmost crystalline 

 strati£.ed masses of ITnst, extending out to the small islet called the 

 Muckle Flugga, on which the most northern lighthouse is built, 

 most probably belong on the whole to the younger gneiss and flag- 

 stone series. 



There may, however, exist, on the western shores of the mainland 

 (which I did not visit), some representatives of the older gneiss. 



