H. Hicks—Pre-Cambrian Volcanos and Glaciers. 489 
fail to be of interest to the geologist, as it constantly presents itself 
to him in all his inquiries. There are also doubtless many who 
are prepared to go with Prof. Ramsay to the length of his conclu- 
sions, and who believe “that from the Laurentian epoch down to 
the present day, all the physical events in the history of the earth 
have varied neither in kind nor in intensity from those which we 
now have experienced ;” whilst others will be inclined to believe 
that though generally they have resembled one another in kind, yet 
that some have varied greatly in their intensity. Most of the 
evidence, hitherto accessible to us, seems to me to favour the latter 
view rather than the former, especially that obtained from researches 
amongst the older groups of rocks. That volcanic action was more 
general and probably more intense in early geological time, and that 
glacial phenomena were less marked at that period than in more 
recent ones would accord, I think, best with our present state of 
knowledge. Volcanic action was undoubtedly prevalent during the 
earliest epochs of which we have any knowledge, and in former 
papers I have given evidence to show that it prevailed in Dimetian, 
Arvonian, and Pebidian times. Prof. Ramsay, however, at p. 7 of 
his address, says that “the oldest volcanic products” of which he 
had any personal knowledge are of “‘ Lower Silurian Age.” 
In a paper published in the Quart. Journ. Geol. Soc. for May, 1878," 
“On the Dimetian and Pebidian Rocks of Pembrokeshire,” I stated 
“that a very large proportion at least of the Pebidian rocks must 
have had a volcanic origin. As, however, they were at first sub- 
aerial, and afterwards suhmarine accumulations, it is evident that 
they must also be partly detrital. The lowest rocks are the agglo- 
merates, and in these I think we have clear evidence of proximity 
to a subaerial voleano surrounded by the ordinary materials of a 
volcanic cone, the largest proportion of the masses being fragments 
of lava which evidently had cooled under atmospheric influences.” 
The thickness of the Pebidian group at St. Davids cannot be less than 
8000 feet, and as by far the greatest proportion of the materials com- 
posing these beds, according to field, and also microscopical examina- 
tion by Mr. 'T. Davies and others, must have had a volcanic origin, 
it is clear that it must have been a time of great volcanic activity. 
From some recent researches it is evident also that at this period in 
geological history, volcanic action was prevalent in many other 
European and American areas, probably far more so than during any 
subsequent period. That the metamorphism of some of the earlier Pre- 
Cambrian rocks was due in part to the disturbances (combined however 
with chemical and other influences) at this time seems also probable, 
and no equal effects seem to have been produced on any group of sedi- 
ments since. Certainly no geologist who has paid much attention 
.of late to the researches going on amongst the older, or Pre-Cambrian 
rocks, can believe with Prof. Ramsay, that the forces as at present 
in action are sufficient to induce equal metamorphic changes in 
groups of such enormous thicknesses, and over such extensive areas, 
as took place in Pre-Cambrian times. That during some other epochs 
1 vol. xxxiv. p. 160, 
