518 Dr. H. Hicks — Pre- Cambrian Rocks in Conglomerates. 



and the mixture of broken quartz and felspar of which some of the 

 beds are almost entirely composed, could only have been derived 

 from the underlying granitoid rocks (Dimetian). Near Llanfaelog 

 and Llanerchymedd in Anglesey, very large pebbles of the under- 

 lying granitoid rocks are abundant in the ovei'lying Cambrian 

 Conglomerates and at Twt Hill, near Carnarvon, the matrix and 

 many of the pebbles must undoubtedly have been derived from the 

 underlying granitoid rocks. 



The so-called Torridon conglomerates and sandstones in Eoss 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. 



The presence of pebbles of granitoid rocks, quartzites, quartz- 

 schists, etc., 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 j'ielded pebbles to the newer pre- 

 Cambrian rocks (Pebidian) and afterwards to the basal Cambrian 

 conglomerates. I maintain 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 granitoid rocks, such 

 as those we have classed in Wales under the name 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 point of age between these 

 granitoid rocks and the Pebidian series. The pre-Cambrian periods 

 therefore, which I have defined by the terms Dimetian, Arvonian, 

 and Pebidian, are easily recognizable, and seem to have succeeded 

 one another in that order. 



In my early papers I stated that the Dimetian granitoid rocks 

 were probably of metamorphic origin. Since then I have been led 

 to change this opinion, and to admit that these peculiar granitoid 

 rocks of early pre-Cambrian age, exposed in several areas in North 

 and South Wales, must have had an igneous origin. There is no 

 evidence to show that there are any rocks of sedimentary origin in 

 Wales which can be classed as belonging to that period. The 

 quartzites, quartz-schists, and porcellanites found in Anglesey and 

 elsewhere were probably derived by denudation from these granitoid 

 rocks, and should be classed, as suggested by me in papers in 1880 

 and 1881, as belonging to the Arvonian period. Some peculiar 

 acid volcanic rocks found in Carnarvonshire and Pembrokeshire 

 belong also to this period, for fragments of these rocks, in the 

 condition in which they are now found, occur abundantly in associa- 

 tion with the pebbles of quartzites, quartz-schists and porcellanites 

 in the Pebidian agglomerates. 



