CHAP. VIII 
DALRADIAN SCHISTS 
121 
area of Lewisiaii gneiss foriuiiig the chain of the Outer Hebrides is investi- 
gated it may possibly supply examples of a similar series of ancient volcanic 
masses. 
ii. THE DALRADIAN OR YOUNGER SCHISTS OF SCOTLAND 
We now come to one of the great gaps in the geological record. Tlie 
Lcwisian gneiss affords us glimpses of probable volcanic activity at the very 
beginning of geological history. An enormous lapse of time, apparently 
unrepresented in Britain by any geological record, must be marked by the 
unconformability between the gneiss and the Torridon Sandstone. Another 
prodigious interval is undoubtedly shown by the Torridonian series. Neither 
this thick accumulation of sediment nor the Cambrian formations, which to 
a depth of some 2000 feet overlie tlie Torridon Sandstone, have yielded 
any evidence of true superficial eruptions, though they are traversed by 
numerous dykes, sills and bosses. The age of these intrusive masses 
cannot be precisely fixed ; a large proportion of them is certainly older 
than the great terrestrial displacements and concurrent metamorphism of the 
North-West Highlands. 
While from the Lewisiaii gneiss upward to the highest visible Cambrian 
platform in Sutherland, no vestige of contemporaneous volcanic rocks is to 
be seen, the continuity of the geological record is abruptly broken at the 
top of the Durness Limestone. By a series of the most stupendous dis- 
locations, the rocks of the terrestrial crust have there been displaced to 
such a degree that portions have been thrust westward for a. horizontal 
distance of sometimes as much as ten miles, while they have been so 
crushed and sheared as to have often lost entirely their original struc- 
tures, and to have passed into the crystalline and foliated condition of 
schists. Portions of the floor of Lewisian gneiss, and large masses of the 
Torridon Sandstone, which had been buried under the Cambrian sediments, 
have been torn up and driven over the Durness Limestone and quartzite. 
Though much care has been bestowed by the officers of the Geological 
Survey on the investigation of the complicated mass of material which, 
pushed over the Cambrian strata,- forms the mountainous ground that lies 
to the east of a line drawn from Loch Eribol, in the north of Sutherland, to 
the south-east of Skye, some uncertainty still exists as to the age and 
history of the rocks of that region. For the purposes of this work, 
therefore, the rest of the country eastwards to the line of the Great Glen — 
that remarkable valley which cuts Scotland in two — may be left out of 
account. 
To the east of the Great Glen the Scottish Highlands display a vast 
succession of crystalline schists, the true stratigraphical relations of which 
to the Lewisian gneiss have still to be determined, but which, taken as a 
wliole, no one now seriously doubts must be greatly younger than that 
ancient rock. Murchison first suggested that the quartzites and limestones 
found in this newer series are the ecpiivalents of those of the North-West. 
