436 Dr. H. Hicks — British Pre- Cambrian Rocks. 



quartzo-felspathic rocks, sometimes porplryritic, frequently brec- 

 ciated : and of compact quartzose rocks or hallefiintas, which on 

 microscopical examination have the appearance of incipient gneiss. 

 The strike is usually about N. and S. 



4. The Pebidian. — This being the newest group in the Pre- 

 Cambrian rocks is the least altered in character, and most nearly 

 approaches in strike to the overlying unaltered or Cambrian rocks. 

 It resembles that group in many of its rocks, and on that account 

 was for a time supposed to be identical with it, only that it had 

 undergone local alteration. Now we know that it underlies the 

 latter unconformably, and that the apparent similarity in character 

 is to be attributed to the fact that most of the Cambrian rocks were 

 derived from the denudation of this group. That it was also in a 

 high state of alteration before the Cambrian rocks were deposited 

 upon it is evident from the fact that an abundance of pebbles and 

 masses of it occur in the conglomerates at the base of the Cambrian. 

 It consists for the most part of chloritic, talcose, felspathic, and 

 micaceous schistose rocks ; alternating with massive and slaty green- 

 stone bands, dolomitic limestone, serpentine, lava-flows, porcellanites, 

 breccias and conglomerates. It is traversed also frequently by dykes 

 of granite, dolerite, etc. It is a group of enormous thickness, and is 

 largely distributed over Great Britain. It occurs in many parts of 

 Wales, in Shropshire, and in Charnwood Forest. I found it also 

 last year in the North-west of Scotland, and I have seen specimens 

 of it collected by Mr. James Thomson and others from May, and 

 other of the Western Islands. Dr. Hunt recognized it also along 

 the Crinan canal, and in the vicinity of Loch Foyle, in Ireland. It 

 is probably represented in America by the Huronian Group. The 

 prevailing strike is N.N.E. to S.S.W., or from this to N.E. and S.W. 



The conglomerates at its base are largely made up of masses 

 derived from the Arvonian ; and it is undoubtedly, at most of the 

 points examined, unconformable to that group. 



These four divisions comprise at present all that we know of the 

 Pre-Cambrian rocks in Great Britain ; but it is probable that as 

 fresh discoveries are being made others will still be found. In any 

 case the field is a wide one, and it has at present only been barely 

 touched, though the results have been very considerable. Many, 

 indeed the majority of the rocks found in these four divisions were 

 until recently supposed to be either intrusive masses, or altered sedi- 

 ments belonging to toler-ably recent times. The success obtained of 

 late in their separation and recognition has been largely due to 

 microscopical examination, for the first application of which to that 

 purpose we are mainly indebted to Mr. Sorby. In addition to this, 

 it is becoming more and more an acknowledged fact that the meta- 

 morphism of great groups of rocks does not take place as readily as 

 was formerly supposed, but that some special conditions, such as do 

 not appear to have prevailed over this area since Pre-Cambrian 

 times, were necessary to produce so great a result. 



