Dr. H. Hiclis — British Pre- Cambrian Rocks. 435 



In former communications I have attempted to classify these rocks 

 as exhibited in special districts, and I have usually found, when 

 examining new areas, that the order which I had previously made 

 out, as far as that evidence went, was almost invariably recognizable 

 there also. It is this fact that has mainly tempted me to bring for- 

 ward this paper to endeavour to show that a classification generally 

 applicable to the British Islands is even now possible. 



Four well-defined groups are made out, and to each of these names 

 have already been given. 



They occur in the following ascending order : — 1. Lewisian; 

 2. Dimetian ; 3. Arvonian ; and 4. Pebidian. 



1. The Leioisian, so named by Sir E. Murchison to indicate the 

 crystalline rocks of the Hebrides and North-western Highlands of 

 Scotland, is here retained for the oldest group at present recognized 

 in Britain, and largely developed in the Hebrides. It is found also 

 in parts of the Malvern chain, and the North-west of Ireland ; and 

 possibly also in Anglesey. The prevailing rocks in this group are 

 massive gneisses, in which hornblende and a reddish felspar are the 

 chief ingredients, and quartz, chlorite, and mica, but sparingly 

 present. They are usually of a dusky red, grey, or dark colour. 

 Sometimes almost a pure hornblende rock is found. The strike in 

 these beds is usually E. and W. or some point between that and 

 N.W. and S.E. 



2. The Dimetian. — This group is largely developed in Wales, as 

 at St. Davids, Carnarvon, Rhos Hirwain and Anglesey. It has been 

 found by Dr. Callaway in Shropshire, and I have recently seen it in 

 the Malvern chain, especially in the Worcester Beacon. I noticed it 

 also last year in large development at Ben Fyn, Loch Maree, and 

 near Gaerloch in Boss-shire, as well as at several other points in the 

 North-western Highlands of Scotland. The prevailing rocks in this 

 group are granitoid and quartzose gneisses, with pinkish, flesh- 

 coloured, or white felspar; and with limestones, micaceous, and 

 occasionally chloritic and hornblenclic bands. Brecciated beds also 

 occur in which bits of the older Lewisian gneiss are sometimes 

 found. The strike is generally N.W. and S.E. or from this to N. 

 and S. It evidently overlies the Lewisian unconformably in the 

 areas where both have hitherto been found associated ; and its highly 

 quartzose character and lighter colour generally is in marked 

 contrast to most of the members of that group. 



3. The Arvonian. — At the last meeting of the British Association 

 I mentioned, for the first time, the discovery, or rather the separa- 

 tion of this group. It is largely developed in Pembrokeshire and 

 Carnarvonshire. It occurs also in Anglesey and Shropshire, and I 

 have recently found it at the base of the Harlech Group in the heart 

 of the Harlech Mountains. I have seen masses of it also from the 

 Orkneys, and it probably occurs both in the Western Islands, and 

 in the Grampians of Scotland. It is the great halleflinta group of 

 the Swedish geologists, and the petrosilex group (Hunt) found so 

 largely developed in North America. It is chiefly made up of 



