CHAP. VIII 
LEWISIAN GNEISS 
119 
biotite-diorite with macro-poihilitic plagioclase ; 4. G-ranites and gneissose 
granites (liiotite-granite with luicrocline) ; 5. Pegmatites (microcliiie-ij^uartz 
rocks with a variable amount of oligoclase or albite).^ 
Distinct evidence of a sncces.sion of eruptions can be made out among 
these rocks. By far thp largest proportion of the dykes consists of basic 
materials. The oldest and most abundant of them are of plagioclase-augite 
rocks, whicli, wliere uncruslied, differ in no essential feature of structure or 
composition from the dolerites and basalts of more modern periods, though 
they have been plentifully changed into epidiorite and hornblende-schist.' 
They present, too, most of the broad features that characterize the dykes 
of later times — the central more coarsely-crystalline portion, the marginal 
band of finer grain, passing occasionally into what was probably a basic 
glass, and the transverse jointing. They belong to more trhan one period 
of emission, for they cross each other. They vary in width up to nearly 
200 -feet, and sometimes run with singular persistence completely across 
the whole breadth of the strip of gneiss in the west of Sutherland and 
Boss. Dozens of dykes have been followed liy the G-eological Survey for 
distances of ten or twelve miles. 
Later in time, and much less abundant, are certain highly basic dykes 
— peridotites with schistose modifications — which cut across the dolerites 
ill a more nearly east-and-west direction. There are likewise occasional 
dykes of peculiar composition, which, as above stated, have been distinguished 
by Mr. Teall as microcline-mica rocks and biotite-diorite. 
I^ast of all comes a group of thoroughly acid rocks — varieties of granite 
and pegmatite — which form intrusive sheets and dykes. The granites 
contain biotite with microcline, and are sometimes gneissose. The pegma- 
tites are mierocline-rpiartz rocks with a variable amount of oligoclase or 
idbite. These dykes coincide in direction with the basalts and dolerites, 
but tliey are apt to run together into belts of granite and pegmatite, some- 
times 1500 feet broad. 
Up to the present time no evidence has been found of any superficial 
Outpouring of material in connection with this remarkable series of dykes 
111 the Lewisian gneiss. That they may iiave been concomitant with true 
volcanic eruptions may be plausibly inferred from the close analogy which, 
in spite of their anticpiity and the metamorphism tliey have undergone, they 
still present to the system of dykes that forms a part of the great Tertiary 
volcanic series of Antrim and the Inner Hebrides. The close-set fissures 
running in a W.H.W direction, the abundant ujirise into these fissures of 
basic igneous rocks, followed by a later and more feeble extravasation of 
acid material, are features which in a singular manner anticipate the 
volcanic phenomena of Tertiary time. 
There can be no cpiestion as to the high antirpiity of these dykes. 
They were already in place, before the advent of those extraordinary vertical 
lines of shearing which liave so greatly affected both the gneiss and the 
' Annual Rcpirrt of Heoloyical Survey for 1895, p. 18 of reprint. 
“ See Mr. Teall, Quart. Juurn. (Icol. Soe. vol. xli. (ISS.")), p. 133. 
