PROCEEDINGS OE THE GEOLOGICAL SOCIETY. 



The Malvern chain may also represent the Lewisian ; and also the 

 highlands of North Ireland as developed in Donegal. 



Dimetian*. — This group is extensively developed in many parts of 

 South and North Wales ; it occurs in Shropshire, and, according to Dr. 

 Hicks, is known in the Worcester Beacon of the Malvern chain ; it is> 

 also said by Hicks to occur at Pen Fyn, Loch Maree, and near Gaer- 

 loch in Eoss- shire, and in other areas in the N.W. Highlands. 



Unlike the Lewisian type, the Dimetian contains impure chloritic 

 limestones and serpentinous bands (not of organic origin so far as we 

 know). The chief minerals are flesh-coloured and white felspar, and 

 chloritic and hornblendic bands ; and the rocks are highly quartzose. 

 Fragments of the older Lewisian gneiss are occasionally found im- 

 bedded in the gneiss, thus determining the unconformability of the- 

 Dimetian group, through derivation and denudation. Yiridite occurs 

 more or less throughout, giving a tinge to the rock. At St. David's 

 the ridge formed by these rocks, according to Dr. Hicks, averages from 

 2000 to 3000 feet in thickness. " Cambrian conglomerates rest im- 

 mediately upon the Dimetian rocks ; and the highest members of the 

 Harlech group strike up against the Dimetian ridge;" and both 

 sides are flanked throughout by the uncomformable Pebidian group. 

 At St. David's the rocks chiefly composing the Dimetian series are 

 compact crystalline quartz, chloritic schists, and indurated shales ; 

 the quartz in places occurs almost perfectly crystalline. Massive 

 beds of calcareous shale and dolomitic limestone occur. et As much as 

 20 per cent, of carbonates have been determined on analysis by 

 Mr. Hudleston, and in addition 0*5 per cent, of phosphoric acid ; 

 hence it is more than probable that the lime was deposited by 

 organic aid " (Hicks). The Pebidian rocks are strongly contrasted 

 by their bedded and shaly character. Dr. Hicks has not discovered 

 any traces of organic life in the inferior limestones, of which five 

 beds occur. 



Dr. Hicks estimates the thickness of the St. David's Dimetian at 

 15,000 feet. Several intrusive dykes traverse the series ; and their 

 injection took place before metamorphism had commenced. They are 

 fine-grained altered dolerites, columnar in structure, with the columns 

 at right angles to the plane of the dykes. 



Arvonian. — In 1878 Dr. Hicks discovered in Pembrokeshire new 

 areas of Pre-Cambrian rocks of a totally different character from 

 the Dimetian and Pebidian which he had previously described. 

 The rocks of this Arvonian group are marked on the Geological 

 Survey map as intrusive felstones; they occur in three or four 

 isolated masses of considerable extent. Their strike is from north 

 to south, and discordant to those of the newer rocks and the under- 

 lying Dimetian. 



The wild mountain-region of Plymstone is mainly composed of 

 the Arvonian group. Dr. Hicks finds Lower Cambrian and Lingula- 

 flag rocks resting unconformably upon the " Arvonian " along their 



* " Dimetia," the ancient name for a kingdom which included this part of 

 Wales. Vide Hicks, "On the Pre-Cambrian (Dimotian and Pebidian) Bocks of 

 St. David's," Quart. Journ. Geol. Soc. vol. xxxiii. p. 229 (1877). 



