II2 
PRE-CAMBRIAN VOLCANOES 
BOOK II 
form the whole chain of the Outer Hebrides, and likewise extend as an 
irregular selvage along the western margin of the counties of Sutherland 
and Eoss. The lowest known platform of the fossiliferous formations has 
there been discoveretl and has been traced for a distance of more than 100 
miles. From this definite horizon, the high antiquity of all that lies 
lielow it is impressively demonstrated. The accompanying diagram (Fig. 
do) will explain the general relations of the various geological formations 
of the region. 
In certain dark shales (h) which occupy a well-defined and readily-traceable 
position among the rocks of Sutherland and Eo.ss, numerous specimens of 
the trilobite genus OleneUus, together with other fossils, have heen found. 
By common consent among geologists, the zone of rock in which this genus 
appears is taken as the lowest stage of the Cambrian system. In Britain 
it marks the oldest known group of fossiliferous strata — the platform on 
which the whole of the Palffiozoie systems rest. 
From the definite geological e]M)ch indicated by this platform, we can gO' 
Fju. 35. — Diagi’ain illustrating tlie stratigrapliical relations of the pre-CainbrUui and Canihriau rocks, 
of the North-west Higlilainls of Scotland. 
c, Durness Limestones, with Upper Camhriun ami perhaps Lower Silurian fossils, 1500 feet, top nowhere seen, 
/q Serpnlite gi’ib and “ fiicoiil ” shales, 70 to SO feet, containing the Oli:ndlu!i-7.one. a, Quartzite, witli 
abundautanuoliil tubes, about 600 feet. II. Red Sand.stoiies and Conglomerates, sometimes 8000 feet or moie 
(Torridonian). 1. Gneiss with dykes, etc. (Lewisian). 
backward into pre-Cambr’ian time, and realize in some measure how pro- 
digious must be the antiquity of the successive groups of rock which emerge 
from beneath the base of the Paheozoic systems. Nowhere is this antiquity 
more impressively proclaimed than in the north-west of Scotland. From 
below the OUnellus-mwe. with its underlying sheets of quartzite {a), a thick 
group of dull red sandstones and conglomerates (II.) rises into a series of 
detached conical or pyramidal mountains, which form one of the most 
characteristic features in the scenery of that region. As this detrital forma- 
tion is well developed around Loch Torridou, it has been termed Torridoniau. 
It attains a thickness of at least 8000 or 10,000 feet, aud is traceable all 
the way from the extreme northern headlands of Sutherland to the southern 
cliffs of the island of Eum. 
In judging of the chronological significance of the geological structure 
of the north-west of Scotland, we are first impressed by the stratigraphical 
break between the base of the Cambrian system and the Torridonian 
deposits below. This break is so complete that here and there tlie thick 
