14 
PRE-CAMBRIAN VOLCANOES 
BOOK 11 
in full detail that their true nature and history have begun to be under- 
stood/ 
The researches of the Survey have shown the so-called Lewisian 
gneiss to comprise the following five groups of rock : 1. A group of 
various more or less handed and foliated rocks which form together the 
oldest and chief part of the gneiss (Fundamental complex) ; 2. Highly basic 
dykes cutting the first group ; 3. Dykes and sills of dolerite, epidiorite and 
hornhlende-schist ; 4. A few dykes of peculiar composition ; 5. Gneissose 
granite and pegmatite. 
The first of these groups, forming the main body of the gneiss, has 
been critically studied on the mainland from Cape Wrath to Skye. But 
its development in the Outer Hebrides has not yet been worked out, 
althougli the name “ Lewisian ” was actually taken from that chain of 
islands. So far as at present known, however, the gneiss of the Hebrides 
repeats the essential cliaracters of that of the mainland. 
Mr. Teall, as the result of a careful investigation in the field and witli 
the microscope, has ascertained that on the mainland hetw'een Skye and 
Cape Wrath the rocks of the “ fundamental complex ” are essentially com- 
posed of olivine, hyperstheue, augite (including diallage), hornblende, 
biotite, plagioclase, orthoclase, microcline and quartz. He has further 
observed that these minerals are associated together in the same manner as 
in peridotites, gabhros, diorites and granites. Treating the rocks in 
accordance with their composition and partly with their structure, hut 
excluding theoretical considerations, he has arranged them in the following 
five subdivisions: — 
1. Eocks coinjioscd of feiTO-iiiagiiesiaii miiicrals, witlioiit ielspar or quartz — Pyroxe- 
nites, Honihlendites. 
2. Eocks ill wliicli pyro.xciics are tlie doiiiiiiatiiig ferro-magiicsian constituents, felspar 
always l)eing in-esent, soinetinies quartz ; A, Without quartz, Hyperstliene- 
augite - rocks (jiyroxene granulites; rocks of the Baltiniore-gahhro type) and 
augite-rocks (gabhros) ; B, With quartz, Augite-gnei.ss. 
:i. Eocks in which honihlende is the prevalent ferro-iuagnesian constituent : A, With- 
' out quartz, or containing it only in small quantity ; rocks basic in cnmposition ; 
(a) massive or only slightly foliated (Aiuphiholites, iis ejiidote-ainpliiholite, 
zoisite- amphibolite, garnet-ampliibolite) ; (6) foliated (Hornblende -schist). B, 
With quartz ; rocks intermediate or acid in composition : (a) with compact 
horid)lende and a granular structure (Hornblende-gneiss in'oper) ; Qi) with horn- 
blende occurring in iibroits or other aggregates ; (c) with compact hornblende 
and a more or less grannlitio structure (Granulitic hornblende-gneiss). 
4. Eocks in which biotite is the predoiniiiant ferro-inagnesian constituent ; felspar and 
quartz both present : ia) Biotite occurring as independent plates or in aggre- 
gates of two or tlirce large indii iduals (Biotite-gneiss) ; (?)) Biotite occurring in 
aggregates of nuiueroiis small individuals (rare ly])e) ; (c) Biotite occurring ns 
independent plates in a granulitic structiun. 
.5. Eocks in which muscovite and biotite are present, together with felspar and quartz 
— Mu8covite-taotite-gneis,s. These, though not forming a well-defined natural 
' See the Report of this Survey work by Messrs. Peach, Horne, Gunn, Clough, Cadell and 
Hinxman, Quart. Journ. Geol. Hoc. vol. xliv. (1888), pp. 378-441 ; and Annual Reports of Director- 
General of the Geological Survey in the Report of The. Science and Art Pepartment for 1894, p. 
279, and 1895, p. 17 of reprint. The general area of the gneiss is shown in Maji 1. 
