On the Crystalline Rocks of the North-west Highlands. ^11 



he intended to point out the existence of much igneous rock causing 

 a great separation between the fossiliferous Lower Silurian and what 

 he termed the " Upper or Eastern Gneiss", it became necessary that 

 the Director- General of the Geological Survey should revise his own 

 conclusions, which had affirmed the existence of an unbroken ascend- 

 ing succession. With this view, he induced Professor Ramsay to 

 accompany him to the north-west of Sutherland, where they spent a 

 month. There they not only saw no reason to depart from any of 

 the views already pul^lished by Sir R. I. Murchison, but were enabled 

 to strengthen them by laying down on a map a more correct outline 

 of the formations than had hitherto been traced, by marking the 

 principal faults, and by indicating clearly the transition upwards 

 from the known Lower Silurian rocks into a superior micaceo-quart- 

 zose series (or the so-called "younger gneiss"), which is entirely 

 dissevered from the Old or Fundamental Gneiss. They further as- 

 certained that, whenever eruptive rocks occurred, they did not in- 

 terfere with or derange this ascending conformable Lower Silurian 

 succession. 



Professor Harkness, who has visited the west of Sutherland since 

 the Meeting of Aberdeen, and who has gone over the principal or 

 typical sections in question, has confirmed these views, and has trans- 

 mitted to the author additional details, which were cited in the pre- 

 sent communication. 



The characters of the " Fundamental Gneiss" which occupies the 

 Western Coast were again dwelt upon to show that its strike from 

 N.N.W. to S.S.E. was essentially distinct from that of the so-called 

 " Upper Gneiss". The latter, as well as the Lower Silurian rocks 

 on which it reposes, ranges from N.N.E. to S.S.W., and. having a 

 prevalent south-easterly dip, is entirely unconformable to the Old or 

 Fundamental Gneiss, which is shown by the author to be the oldest 

 stratified rock yet discovered in the British Isles, and to be of the 

 same age as the Laurentian System (of Logan) in North America. 



After some additions to the data respecting the Cambrian Sand- 

 stones and Conglomerates, which, occupying the loftiest mountains 

 of the west coast, rest unconformably on the upturned edges of the 

 older gneiss, new sections were exhibited to show the perfect con- 

 formability of all those strata which, lying transgressiyely on such 

 Cambrian rocks, were referred, as a whole, to the Lower Silurian rocks. 



In the district of Assynt there are clear evidences demonstrating 

 that the limestones are surmounted by quite as thick a mass (Ben 

 More in Assynt, etc.) of quartz-rock as that which lies beneath them ; 

 both the Lower and Upper Quartz being characterized by numerous 

 annelide-tubes. The igneous rocks that are there intercalated with 

 the limestone, or appear in the overlying quartz-rock, produce no de- 

 rangement of the order. 



Attention was called to the former observation of the author, that 

 the whole of this quartzose and calcareous series is overlaid by an- 

 other limestone, which, in its turn, passes up conformably into other 

 crystalline rocks, chiefly micaceous schists and flagstones, which are 

 represented, as before said, in all geological maps in tiie same colours 

 as the Old or Fundamental Gneiss. 



