804 REPORT—1890. 
The author states that the so-called Torridon conglomerates and sandstones, 
in Ross and Sutherland, contain abundant evidences to show that most of the 
materials were obtained from the rocks upon which they now rest, after the latter 
had assumed their present condition. 
He claims that the presence of pebbles of granitoid rocks, quartzites, quartz- 
schists, &c., in all the areas, proves clearly that some granitoid rocks were exposed 
to denudation on a large scale in many areas, in very early Pre-Cambrian times, 
for materials derived by denudation from the latter rocks must have been formed 
into quartzites, porcellanites, and schists (Arvonian rocks) in early Pre-Cambrian 
times. By subsequent denudation these yielded pebbles to the newer Pre-Cambrian 
rocks (Pebidian), and afterwards to the basal Cambrian conglomerates. 
The author maintains therefore that the Pre-Cambrian rocks contain evidences 
of successive periods of elevation and depression, and probably of volcanic activity, 
and that the tendency of the evidence is undoubtedly to show that some of the 
granitoid rocks (Dimetian) are amongst the very oldest of the Pre-Cambrian rocks 
which are now found exposed, and that some quartzites, porcellanites, and schists 
occupy an intermediate position in age between these granitoid rocks and the 
Pebidian series. The Pre-Cambrian periods, therefore, which have been defined by 
the author by the terms Dimetian, Arvonian, and Pebidian, are easily recognisable 
whether the names be accepted or not. 
3. The Effects produced by Earth-movements on Pre-Cambrian and Lower 
Paleozoic Rocks in some Sections in Wales and Shropshire.’ By Hunry 
Hicks, M.D., F.R.S., F.G.S. 
The author in this paper gives examples to show the powerful influences exerted 
by earth-movements in producing changes in the rocks, and in obliterating the 
evidences of succession in the disturbed areas in Wales and Shropshire. He points 
out that the difficulties experienced by geologists who examine these areas for the 
first time are mainly due to their being unable or unwilling to recognise the extra- 
ordinary effects produced by these earth-movements, and especially the complica- 
tions due to faults and thrusts. Frequently, he says, portions of the Pre-Cambrian 
rocks have been forced in among the Lower Paleozoic rocks so as to appear either 
to be parts of the series or to be intruded into it. In other places they have been 
made to appear to overlie much newer beds. A section across the St. David’s pro- 
montory shows an arch of Cambrian rocks, and of Arenig beds containing great 
masses of igneous rocks, probably portions of sheets in the forms of Laccolites, all 
bent over a core of Pre-Cambrian rocks, and repeatedly broken on the west side by 
thrust-moyvements, causing newer beds to be driven over beds of various horizons, 
in some cases many thousands of feet apart in the succession ; whilst on the east 
side the limb is broken by reversed faults, so as to make the beds appear to dip 
under the Pre-Cambrian rocks. Again, in the Pre-Cambrian core itselt the Pebidian 
rocks are not only sheared to an enormous extent, but are also made, on the south 
side, by reversed faults, to appear to lie under paris of the granitoid rocks (Dime- 
tian) ; one result of these mechanical movements being to make the Dimetian look 
as if intruded into the Pebidian beds, whilst in reality it is everywhere here bounded 
by faults, as the result of repeated earth-moyements in Pre-Cambrian and subse- 
quent periods. The author also shows that very similar results have taken place 
in the sections between the Menai Straits and the Snowdon district, where not only 
do the Cambrian rocks appear to underlie the Pre-Cambrian, but at one point even 
Arenig beds are made to dip under both, 
The author states that in a section in Shropshire, extending from the Longmynd 
across Caer Caradoc, Lower Paleozoic rocks are faulted so as to appear to underlie 
the Pre-Cambrian rocks of Caer Caradoc; whilst on the east of Caer Caradoc, as 
the result of thrust-moyements, great thicknesses of the lower beds have been 
hidden by much newer ones. He mentions that the changes which have been 
produced in the rocks themselves are also very marked. The granitoid rocks give 
' Published in extenso in Geol. Magazine for December 1890. 
~~ 
