ii8 
PRE-CAMBRIAN VOLCANOES 
ROOK n 
accompanied l)y any manifestations of superficial volcanic action has not 
yet been determined. Eut we know that they were followed l)y a succes- 
sion of dyke-eruptions, to which, fijr extent and variety, there is no parallel 
in the geological structure of Britain, save in the remarkable assemblage 
of dykes belonging to the Tertiary volcanic period ’.(Fig. .36). 
Taken from Sheet 107 of the Geological Survey of Scotland on the scale of one inch to a mile. The white ground 
('^) marks the general body of the Lewisiaii gneiss. Tliis is traversed by <lykes of dolerite (B), which are cut by 
later dykes of highly basic material (]>eridotite, pjcrite, etc., P). The gneiss and its system of dykes is overlain 
uncoiiformably by the nearly liorizontiil Torridon Sandstone (/), winch is injected by .sheets of oligoclase- 
porphyry (F). 
For the production of these dykes a series of fissures was first opened 
tlirough the fundamental complex of the gneiss, having a general trend 
from E.S.E. to W.N.VV., running in parallel lines for many miles, and so 
close together in some places that fifteen or twenty of them occurred 
within a horizontal space of one mile. The fissures were probably not all 
formed at the same time ; at all events, the molten materials that rose in 
tliem exhibit distinct evidence of a succession of npwellings from the 
igneous magma below. 
Considered simply from the petrographieal point of view, the materials 
tliat have filled the fissures have been arranged by Mr. Teall in the follow- 
ing groups: 1. Ultra-basic dykes, sometimes massive (peridotites), some- 
times foliated (talcose schists containing carbonates and sometimes gedrite) ; 
2. Basic dykes which where massive take the forms of dolerite and epi- 
diorite, and where foliated appear as hornblende-schist, the same dyke often 
presenting the three conditions of dolerite, epidiorite and hornblende-schist ; 
3. Ifykes of peculiar composition, comprising microcline-mica rocks and 
' Quart. Jmirn. Qcvl. Soc. vol. xliv. (1888), p. 389 ct set/. 
