CHAP. VIII 
THE LEWIS JAN GNEISS 
1 1 1 
of the terms applied to the fossiliferous formations. Yet the convenience of 
possessing such a general descriptive epithet is obvious. 
Until much more knowledge of the subject has been gained, any 
terminology which may he proposed must he regarded as more or less 
provisional. The comprehensive term “ pre- Cambrian ” may he usefully 
adopted as a general designation for all rocks older than the base of the 
Cambrian system, irrespective of their nature and origin. Already it is well 
known that under this term a vast series of rocks, igneous and sedimentary, 
is included. In some regions several successive formations, or systems of 
formations, may be recognized in this series. But until some method has 
been devised for determining the stratigraphical relations of these formations 
in different regions, it would seem safest not to attempt to introduce general 
names for universal adoption, but to let the sequence of rocks in each distinct 
geological province be expressed by a local terminology, ibis caution is 
more especially desirable in the case of sedimentary depo.sits. We may 
surmise as to the equivalence of the rocks called Huronian, d orridonia]i 
and Longmyndian, but whilst so much is mere conjecture, it is certainlj 
injudicious to transfer the local names of one province to the rocks of another. 
The only relaxation of tliis general precaution which I tliink may at 
present be made is the adoption of a common name for the oldest type of 
gneisses. The term “ Archaean ” has been applicnl to these rocks, and if it 
is used simply to express a common petrogra])hical type, occupying the 
lowest horizon in the stratigraphical series of a country, it has obvious 
advantages. But I would still retain the local names as sulwrdinate terms 
to mark'’tlie local characteristics of the Archaean rocks of each province. 
Thus the “ Laurentian ” rocks of Canada and the “ Lewisian ” rocks of 
Scotland are widely-separated rej)resentatives of the peculiar stratigraphical 
series whicli is known as Archaean. 
The pre-Cambrian rocks of Britain include several distinct systems or 
groui)s. How far those of even one part of this comparatively limited 
region are the proper equivalents of those of another and distant part is a 
problem still unsolved. Hence each distinct area, with its own type of rocks, 
will here be treated by itself. The following rock-types will be described : 
I. The Lewisian (Archaean) ffneiss; li. The Younger (Dalradian) Schists 
of Scotland; iii. The Gneisses and Schists of Anglesey ; iv. The Uriconian 
Grouy) ; v. The Malvern Group ; vi. The Charnwood i orest Group (see Map I.). 
i. THE LEWISIAN (ARCHAEAN) GNEISS 
The British Isles are singularly fortunate in possessing an admirable 
development of pre-Cambrian rocks. These ancient masses rise up in 
various parts of the islands, but the region where tliey are most extensively 
displayed, and where their stratigraphical position and sequence are most 
clearly shown, lies in the north-west of Scotland.^ In that territory they 
' Tliese rocks liavc been the subject of much discussion, but geologists arc now agreed as to 
tlieir sueoession and structure. A full summary of the literature of the controversy regaiHiug 
them will be found in the Quarterly Jerurual of the Geoloyieal Society, vol. .\liv. (1888), ]i. 378. 
