584 REroRT— 1880. 



4. 0?i some Pre-Camhrian Bocks in the Harlech Mountains, Merionethshire, 

 By Henkt Hicks, M.D., F.G.8. 



During an excursion into the Harlech Mountains in the summer of last year, I 

 recognised, near the centre of the well-known anticlinal of Cambrian rocks, another 

 group of rocks which appeared to me to underlie the former, and to be part of a 

 pre-existing formation. On further examination, I noticed also that many of the 

 fragments in the conglomerates at the base of the Harlech grits seemed to be iden- 

 tical with the rocks below, and to have been derived from some such pre-existing 

 group. Subsequent microscopical examination of some of the fragments and of the 

 underlying rocks tended strongly to confirm this ■^iew. In order, however, to satisfy 

 myself more fully on this poiut, I revisited the area this summer, accompanied by 

 my friends, Professor Hughes, Mr. Tawney, and Dr. R. D. Roberts, and the result 

 has been to entirely confirm my previous conclusions. This discovery is of consider- 

 able importance, as it enables us to compare the thickness of the Cambrian rocks 

 of North Wales more satisfactorily than has been hitherto possible with those of 

 South Wales, and to realise more clearly the early physical conditions of the areas. 

 Hitherto it seemed doubtful what the actual thickness of the Harlech group could 

 be, and very diiferent estimates have been given. It now becomes possible to give a 

 perfectly correct estimate, and it is satisfactory to find that it approximates far 

 more nearly with that made out in other Wekh areas than was previously sup- 

 posed. The points where these older rocks come to the surface mainly occur along 

 a line running nearly due N. and S. from Llyn-Cwmmynach to about two miles to 

 the S.W. of Trawsfynnydd. Along this line the anticlinal of Cambrian rocks is 

 considerably broken, and denudation has taken place to a very considerable extent. 

 It is mainly in consequence of this that the pre-Cambrian rocks are exposed. The 

 so-caUed intrusive felstones marked here on the survey maps are part of the pre- 

 Cambrian group, and are not intrusive in the Harlech rocks. They are highly 

 felsitic rocks, for the most part a metamorphic series of schists alternating with 

 harder felsitic bands, probably originally volcanic ashes. They alternate with 

 bands of purplish slates, which I once supposed might have been dropped amongst 

 them by faults, but which I now think also belong to the pre-Cambrian group, as 

 in the Pebidian rocks at St. Davids, and elsewhere. There are also some other 

 exposures of the pre-Cambrian rocks in the adjoining areas, and one very interesting 

 •section was carefully examined by Professor Hughes and myself to the east of the 

 Trawsfynnydd road between Caean Cochion and Penmaen, where the Cambrian 

 conglomerates could be seen resting unconformably upon the older series, and large 

 masses of the latter found plentifully in the conglomerates. 



5. On the Fault Systems of Central and West Cornwall. 

 By J. H. Collins, F.O.S. 



The author remarked that the faults and fault-systems of the district in question 

 "were very numerous, but that they were much more important, on account of their 

 mineral contents, than for their mechanical effect in displacing the strata. After 

 referring to the eight systems of faults — all being mineral veins — which were defined 

 by Mr. Jos. Came in 1818, the author brought forward evidence to prove that no 

 fewer than fifteen distinct fault-systems of as many different ages could still be 

 traced, all having been produced in post-carboniferous times. The fifteen systems 

 were detailed as follows : — 



1. Granite junction faults, the fiUing generally schorlaceous, and often stan- 

 niferous. 



2. The older Elvan faults (the Little Elvan at Polgooth, &c.), more or less 

 granitic in their filling. 



3. The oldest tin lodes (Polgooth, &c.), heaved by the Great Elvan. 



4. Newer Elvan faults (Polgooth, Great Elvan, &c.). 



