398 Dr. H. Hicks—Pre-Cambrian Rocks of Wales 
have persistently denied that there were any Pre-Cambrian rocks in 
Wales. Further researches in other districts in Pembrokeshire and 
in North Wales enabled me to announce that there was evidence to 
show that several other great masses of igneous rocks shown on 
the maps of the Geological Survey as intrusive in Cambrian and 
Lower Silurian strata, were in reality portions of a Pre-Cambrian 
land which had been forced up by great earth movements and after- 
wards exposed to view by denuding agencies. This mass at St. 
Davids and others of a like nature in Anglesey and Carnarvonshire 
had been credited by the Geological Surveyors with producing marked 
metamorphic changes in the rocks which surrounded them, so that 
large areas in their immediate neighbourhood were coloured as 
metamorphosed Cambrian and Silurian strata. These so-called 
metamorphosed rocks, as will be explained further on, I was able 
subsequently to show to be mainly volcanic rocks of Pre-Cambrian 
age. ‘The igneous rocks in Wales, which are now admitted to be 
of Pre-Cambrian age by the majority of the geologists who have 
examined them, are marked on the Geological Survey maps either 
as syenite, granite, felstone or felspathic porphyry. ‘These masses, 
subsequent to their consolidation, have been freely traversed by 
dykes of acid and basic igneous rocks, and the signs of alteration 
which are occasionally seen are due to contact with these dykes. 
Some of the dykes are undoubtedly of Pre-Cambrian age, and they 
show clear indications of having suffered from the movements which 
affected the rocks in Pre-Cambrian times; others are of more recent 
date, and traverse both the Pre-Cambrian and the overlying series. 
Igneous rocks of somewhat similar character to some of the masses 
now classed as of Pre-Cambrian age may be recognized here and 
there as having been intruded into Cambrian and Lower Silurian 
strata; but these do not show evidence of having been crushed or 
foliated to anything like so marked a degree as the similar rocks of 
Pre-Cambrian age. To the granitoid rocks (massive and gneissose), . 
in the year 1877, I gave the name “ Dimetian,” and I classed them 
as the oldest of the Pre-Cambrian rocks in Wales. They are in 
fact the rocks which Sir Archibald Geikie now says are “quite 
comparable in lithological character to portions of the Lewisian 
rocks of the North-West of Scotland.” It is important to note 
that in our earlier papers we classed these masses, as we do now, 
as of igneous origin, maintaining that they were of Pre-Cambrian 
age, and not intrusive in the Cambrian and Lower Silurian strata ; 
but unfortunately the extravagant views subsequently in vogue in 
regard to questions of metamorphism led us, like others, for a time 
to believe that it was possible that even some of these massive 
crystalline rocks might originally have been sedimentary strata. 
When the microscope, however, was more freely used in their 
examination, it became apparent that they could not at any time 
have been sediments, and Professor Bonney, Mr. T. Davies, Dr. 
Callaway, Mr. Teall, and others have now clearly demonstrated 
that they are in the main igneous bosses, some still retaining their 
massive character though showing indications of having been much 
