20 



NATURE 



[November 7, 1895 



plication, and have undergone more or less deformation in the 

 course of the gigantic earth-movements which placed them in 

 their present positions. So far as at present known, no un- 

 moved tracts of the oldest gneiss are to be looked for in the 

 regions of Ross-shire and Inverness-shire yet to be mapped, but 

 there may be many more or less recognisable disrupted masses 

 among the crystalline schists of that region. 



The only area where any of the Lewisian gneiss was mapped last 

 year, on the western or undisturbed side of the great line of dis- 

 placement, lay in the Island Raasay, where Mr. Teall completed 

 the survey of these oldest rocks. As far as the mapping of the 

 north-west Highlands has advanced, the various crystalline rocks 

 older than the Torridon sandstone, and comprised under the 

 general designation of Lewisian gneiss, may be divided into five 

 distinct groups, (i) What has been termed the " fundamental 

 complex," consisting of various more or less banded and foliated 

 rocks which form together the oldest, and chief part of the gneiss. 

 (2) Highly basic dykes cutting the fundamental complex. (3) 

 Dykes and sills of dolerite, epidiorite, and hornblende-schist. 

 (4) A few dykes of peculiar composition. (5) Gneissose granite, 

 and pegmatite. 



Mr. Teall has devoted himself, both in the field and with the 

 microscope in the office, to the patient study of these rocks, and 

 he has at last been able to formulate his views regarding the 

 nature and composition of the various rocks comprised within 

 the first of these sub-divisions, the " fundamental complex." He 

 reports to me that over the greater portion of the area between 

 Skye and Cape Wrath the rocks of the " fundamental complex " 

 have decided affinities, both as regards chemical and minera- 

 logical composition with plutonic igneous products, and his 

 detailed Report, so far as these rocks are concerned, is now 

 fairly complete. They are essentially composed of olivine, 

 hypersthene, augite (including diallage), hornblende, biotite, 

 plagioclase, orthoclase, microcline, and quartz ; and the laws of 

 paragenesis are the same as those which govern the composition 

 of peridotites, gabbros, diorites, and granites. It is rare to find 

 any one petrographical type persistent over a large area. Varia- 

 tions in the relative proportions of the different constituents are 

 almost everywhere recognisable, and these variations may be 

 either abrupt or gradual. One general law appears to have been 

 established as far as the fundamental complex is concerned. 

 Whenever the relative ages of two distinct petrographical types 

 can be clearly ascertained, the more basic is older than the more 

 acid. 



In classifying the rocks, either with reference to structure or 

 composition, difficulties arise in consequence of transitions in 

 various directions. Many schemes have been proposed ; and the 

 following one, based primarily on mineralogical composition, 

 and to a subordinate extent on structure, has been devised by 

 Mr. Teall for descriptive purposes. Theoretical considerations 

 have been excluded, and it is hoped that whatever view may be 

 finally adopted, the broad general facts will be found to have 

 been correctly recorded : — 



I. Rocks composed of ferro-magnesian minerals without fel- 

 spar or quartz. 



(i) Pyroxenites. 



Locality. Scourie, Drumbeg. 

 (2) Hornblendites. 



Same localities as pyroxenites, and also as lumps in 

 the gneiss of many other localities. 



II. Rocks in which pyroxenes are the dominating ferro- 

 magnesian constituents. Felspar always present, and in some 

 cases quartz. 



{A ) Without quartz. 



{a) Hypersthene-augite-rocks. 



(i) With garnet (pyroxene-granuhtes). 



Locality. Scourie. 

 (2) Without garnet (rocks of the Baltimore-Gabbro 

 type). 

 Locality. Scourie, Oruinard. 

 {b) Augite-rocks. Gabbros in structure and com- 

 position, but forming part of the fundamental 

 complex, and often associated with quartz-bearing 

 rocks of a similar character. 

 Locality. Kyle Sku, Loch Inver. 

 {B) With quartz. 



(i) Augite gneiss. 



Locality. Kyle Sku, Loch Inver. 



NO. 1358, VOL. 53] 



HI. Rocks in which hornblende is the dominating ferro- 

 magnesian constituent. 



{A) Without quartz, or containing it only in small quantity. 

 Rocks basic in composition. 

 (a)sRocks massive or only slightly foliated (Amphi- 

 bolites). 



( 1 ) Epidote-amphibolite. 



Locality. Near Stoer. 



(2) Zoisite-amphibolite. 



Locality. Sangomore Bay. 



(3) Garnet-amphibolite. 



Locality. Between Scourie and Laxford Bridge, 

 (b) Rocks foliated : — 

 Hornblende schist. 



Locality. Between Laxford Bridge and Dur- 

 ness, also at Shieldag, Rona, and many other 

 localities. 

 Note. — Many of the hornblende-schists found in 

 the Lewisian gneiss are foliated dykes. 



{B) With quartz. Rocks intermediate or acid in composi- 

 tion, 

 (i) Rocks with compact hornblende and a granular 

 structure. Hornblende-gneiss (proper). 

 Locality. Between Laxford Bridge and Durness, 

 Poolewe, Rona, Raasay, and many other 

 localities. 



(2) Rocks with hornblende occurring in fibrous or other 



aggregates :— 

 Locality. Between Scourie and Loch Inver, 

 especially found in the same area as the 

 augite-gneisses. 



(3) Rocks with compact hornblende, and a more or 



less granulitic structure. Granulitic hornblende- 

 gneiss. 

 Locality. In zones of secondary shear about 

 Loch Inver. 

 IV. Rocks in which biotite is the dominating ferro-magnesian 

 constituent. Felspar and quartz both present. 



( 1 ) Biotite occurring as independent plates or in aggre- 



gates of two or three large individuals. Biotite- 



gneiss (proper). 

 Locality. Between Laxford Bridge and Dur- 

 ness, Rona, Raasay, and many other locali- 

 ties. 



(2) Biotite occurring in aggregates of numerous small 



individuals. 

 Locality. Associated with the augite-gneisses. 

 A rare type. 



(3) Biotite occurring as independent plates. Structure 



granulitic. 

 Locality. In zones of secondary shear. 



While mapping the Lewisian gneiss of Raasay, Mr. Teall 

 observed some curious patches of breccia in that island. One of 

 these descends vertically into the Torridon sandstone, another 

 occupies a similar position in the gneiss. The breccia consists 

 of fragments of Torridon sandstone cemented with calcite, from 

 which small rhombs of clear Iceland spar may be obtained. In 

 one instance where the outlines of the breccia can be more dis- 

 tinctly traced, the ground plan of the mass is nearly semicircular, 

 as if the breccia formed a plug in the Torridon sandstone. 



Some interesting additions have recently been made to our 

 knowledge of the Cambrian rocks of the north-west. Mr. Home 

 has found a band of fossiliferous ironstone, about two feet thick, 

 in the ' ' Fucoid beds " above Auchnashellach Station. The 

 seam is full of 'a small discinoid or linguloid brachiopod. Mr. 

 Macconochie has renewed his search for fossils in the Olenelhis- 

 zone, and has obtained additional specimens not only from the 

 locaHties above Loch Maree and near Dundonnel, but from other 

 outcrops of the same zone further to the north. He has detected 

 fragments of trilobites together with other fossils of the same 

 horizon on both sides of Loch Broom near Ullapool. He has 

 likewise found them where the "Fucoid beds" appear in the 

 River AchuU, the Allatryne Burn, Strath Kaniard, Drumrunie 

 and Knockan. Still further north Mr. Peach has obtained frag- 

 ments of Oknellns from the same horizon at Inchnadamph. 

 The ironstone-bed with brachiopods just referred to has been re- 

 cognised by Mr. Macconochie in Glen Logan and other parts of 



