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640 STRATIGRAPHICAL GEOLOGY. - [Boox VI. — 
_ crystalline diabase are intercalated with the Huronian quartzites. 
In various localities in Wales and England what have been described 
as rhyolitic lavas and coarse agglomerates occur in supposed insular 
areas of Archean rocks. 
Among masses so thoroughly crystalline in structure, crystalline — 
minerals, as may be expected, are specially abundant. Among 
these may be mentioned hornblende, actinolite, tremolite, pyroxene, 
vesuvianite, serpentine, kyanite, graphite, garnet, epidote, apatite, 
tourmaline, wollastonite, zircon, fluor-spar, pyrite, chalcopyrite, 
maenetite, titaniferous iron, and hematite. Some of these minerals 
(iron-ores, hornblende, apatite) occasionally form massive lenticular 
bands as well as run in a diffused form through the limestone or 
gneiss. Certain regions (Sweden, Erzgebirge, &c.) abound in veins 
of metallic ores—gold, silver, copper, lead, &c. 
The largest areas of Archean rocks now exposed at the surface 
are in the northern parts of Europe and North America. Hlsewhere 
they rise as isolated insular spaces surrounded with younger 
formations. 
§ 2—Local Development. 
Britain.—In no part of the European area are these ancient rocks 
better seen than in the north-west of Scotland. Their position there, 
previously indicated by Macculloch and Hay Cunningham, was first 
definitely established by Murchison, who showed that they possess a 
dominant strike to N.N.W., and are unconformably overlaid by all the 
other rocks of the Scottish Highlands. (See Fig. 300.) They form 
nearly the whole of the Outer Hebrides, and occupy a variable belt of 
the western parts of the counties of Sutherland and Ross. Murchison 
proposed to term them the Fundamental or Lewisian Gneiss from the 
isle of Lewis—the chief of the Hebrides. Afterwards he called them 
Laurentian, regarding them as the equivalent of some part of the great 
Laurentian system of Canada. They consist of a tough massive gneiss 
usually hornblendic, with bands of hornblende-rock, hornblende-schist, 
actinolite-schist, eclogite, mica-schist, sericite-schist, and other crystal- 
line rocks. In two or three places they enclose bands of limestone, but 
neither in these nor in any other parts of their mass has the least trace 
of any organic structure been detected. In traversing the western 
sea-board, from Cape Wrath to Loch Torridon, I have ascertained that 
these ancient rocks are disposed in several broad anticlinal and synclinal 
folds, the angles of dip often not exceeding from 30° to 40°, and the strata 
succeeding each other with unexpected regularity, though here and 
there showing great local crumpling. The lower portions of the series 
are on the whole more massive than the upper, and more traversed by 
pegmatite veins. Between Loch Laxford and Cape Wrath this lower 
division has a distinctly pinkish tint from the colour of its abundant 
orthoclase, and the number and size of its pegmatite veins. Some of 
the lowest bands of the formation may be observed along an anti- 
clinal fold north of Loch Inver, where one of the most conspicuous rocks 
is an unctuous sericitic schist. ‘he upper division cannot be sharply 
defined, but is on the whole marked by the relative thinness of its beds, 
