CHAP, vni 
LEW I SI AN GNEISS 
113 
intervening 
mass of sandstones and conglomerates has been nearly or 
removed by denudation before the lowest Cambrian strata were 
Such a discordance marks the passage of a protracted interval 
wholly 
laid down. 
\ifain when tlie composition of the Torridonian rocks is considered, 
further striking evidence is obtained of the lapse, of long periods. The 
sandstones, conglomerates and shales of this pre-Caml:.rian system present 
no evidence of cataclysinal action. On the contrary, tliey hear testimony 
that they were accumulated much in the same way and at the same rate 
as the sulisequent Talseozoic systems. In that primeval period, as now, 
sand and silt were spread out under lakes and seas, were ripple-markei )y 
the agitation of the water, and were gradually buried under other layers 0 
similar sediment. The accumulation of 10,000. feet of such gradually- 
assorted detritus must have demanded a long series of ages.^ Here then, ni 
the internal structure of the Torridonian rocks, there is proof that 111 
passing across tliem, from their summit to their base, we make another vast 
stride, backward into the early past of geological history. 
But when attention is directed to the relations ot the lorridoman strata 
to tlie rocks heneatli them, a still more striking proof of an enormously 
protracted period of time is obtained. Between the two series ot formations 
lies one of the most marked stratigraphiciil breaks in the geolopcal structuie 
of the British Isles. There is absolutely nothing in common between them 
save that the conglomerates and sandstones have been largely nnnle out ot 
the waste of tlie underlying gneiss. The denudation oi the cij sta^ me 
rocks liefore the deposition of any of the Torridonian sediments must have 
been prolonged and gigantic. The more, indeed, we study tlie gneiss, the 
more do we feel impressed by the evidence for the lapse of a vast inteival 
of time, here unrecorded in rock, between the last terrestrial inoi-ements 
indicated by the gneiss and the earliest of the Torridonian sediments. ^ 
In this manner, reasoning backward from the horizon of the OlcneUui- 
zone, we are enabled to form some conception of the vastness of the antiquity 
of the fundamental rocks of the North-west Highlands. The nature and 
origin of these rocks acquire a special interest from a consideration of then 
age. They contain the chronicles of the very beginniiigs of geological 
history, in so far as this history is contained 111 the crust ot the earth 
No part of the geological record is so oliscure as this earliest chapter, but 
we need not here enter further into its difficulties than may be necessary 
for the purpose of understanding wliat light it can be made to throw on 
the earliest manifestations of I'olcaiiic action. 
Under the term Lewisiaii Hneiss (I. in Fig. 35) a series of rocks is com- 
prised which differ from each other in composition, structure and age, 
though most of them possess such crystalline and generally foliated characters 
as may be conveniently included under the designation of gneiss. The com- 
plexity of these ancient crystalline masses was not recognized at the time 
when Miu'chison called them the “Fundamental” or “ Lewisian gneiss. 
It is only since the Geological Survey began to study and map ^ them 
VOL. I 
