THE PRE- CAMBRIAN ROCKS, BRITISH ISLES. i 3 



types of ground where the oldest gneiss forms the surface in 

 Scotland and the west of Ireland. In the Malvern Hills another 

 small knob of somewhat similar material is obviously far more 

 ancient than the Cambrian rocks of that locality. There may 

 possibly be still some further exposures of similar rocks in the 

 south of England, as for instance in southern Cornwall. In 

 Anglesey a series of schists, quartzites and limestones has been 

 included by Mr. J. F. Blake with the coarse gneiss above referred 

 to, and a thick higher group of slates in what he terms the 

 "Monian" system. These schists, quartzites and limestones 

 present a close resemblance to the Dalradian series of Scotland 

 and Ireland, and the quartzites, like those of the Highlands, 

 contain v/orm-burrows. The coarse gneiss, as I have said, may 

 be compared in general character with parts of the Lewisian 

 rocks, so that we seem to have here, as in Ireland, two groups 

 of schistose rocks, and both of these must be much older than 

 the unaltered Cambrian strata which lie above them. 



Along the eastern borders of Wales, there is an interrupted 

 ridge of igneous rocks which were originally supposed to have 

 broken through the older Paleozoic formations, but which now, 

 owing mainly to the labors of Dr. Callaway and Professor 

 Lapworth, are shown to be older than the base of the Cambrian 

 system. These rocks consist of spherulitic and perlitic felsites, 

 with volcanic breccias and tuffs. They are undoubtedly older 

 than the Olenelliis zone. Though the evidence is not quite satis- 

 factory, they may not impossibly lie at the base of a vast mass 

 of sedimentary rocks forming the ridge of the Longmynd. In 

 that case the whole of the Longmynd succession with the 

 volcanic group at its base must be pre-Cambrian and lie uncon- 

 formably below the Olenelliis zone. Dr. Callaway has proposed 

 the name "■Uricoman' for this volcanic group, while the sedimen- 

 tary series has been termed ''Lo/igjnyndian." On the supposition 

 that the unconformability is established, there would here be a 

 vast mass of stratified and partly erupted material forming a pre- 

 Cambrian formation. Whether in that case any portion of this 

 English series is the equivalent of the Torridonian rocks of 



