446 



MR. A. WADE ON THE LLANDOVERY AND [Aug. I9IT: 



■the lowest member of the Valentian, and the unconformity with 

 accompanying conglomerates occurs later. In Gwern-y-brain, the 

 unconformity along with the conglomerate stage appears to occur 

 at, or very Dear, the base of the Valentian. Other points of simi- 

 larity between the two areas have already been mentioDed. 



(6) North Wales. — On visiting the Bala area itself, I found 

 many points of similarity; but, although the work of Thomas 

 Ruddy l established the general succession in this area, the sub- 

 division of the sequence and its exact correlation still remain to be 

 done. The lists of fossils from the Lower Bala Limestone of Bala 

 are closely comparable with those drawn up from the limestone 

 horizon in the Gaerfawr Grits. I have, therefore, chosen the 

 succession worked out by Miss Elles at Conway 2 as presenting 

 the most complete sequence that I could get in North Wales for 

 comparison. Shales predominate in that district, yielding many 

 graptolite horizons, whereas shallower-water conditions existed at 

 Welshpool and graptolites are not so abundant ; and yet there is a 

 good deal of similarity between the palaeontological succession in 

 both areas. 



(7) The Lake District. — Curiously enough, it is when we come 

 to the Lake District and the Scottish (Girvan) area, tbat we find 

 the closest and most complete similarity, especially in lithological 

 characters. 



There are many points of resemblance in the lower part of the 

 sequence developed in the Lake District. The limestone develop- 

 ment in the Gaerfawr Grits seems to correspond very well with 

 the Applethwaite Limestone ; while the position and characters of 

 the Ashgill Shales and Staurocephalus Limestone are very similar 

 to those of the Gwern-y-brain Shales and Limestone. The uncon- 

 formity, seen at the top of the Ashgillian in the. Welshpool area, 

 does not appear to be present in the typical area, and so the Powis- 

 Castle Group can only be correlated on palaeontological grounds. 



(8) Southern Scotland (Girvan area). — Here, again, the 

 sequence is similar. There are some surprising similarities in 

 the palseontoiogical character of the beds : for instance, the 

 abundance of gasteropods at certain horizons in the Caradocian. 

 The Pwll-y-glo Beds probably correspond to the Ardwell Flags, 

 the graptolites present in them indicating that horizon. 



VIII. Igneous Rocks— The Welshpool Dyke. 



The physical characters of the Welshpool Dyke have been very 

 fully described by Murchison [1]. I have little to add to his 

 account, so far as its texture and appearance are concerned. It 

 illustrates very well the internal structure of a large dyke, and is 



1 Q. J. G. S. vol. xxxv (1879) pp. 200-208. See also A. J. Jukes-Browne, 

 Student's Handbook of Stratigraphical Geology ' 1902, pp. 109-12. 



2 Q. J. G. S. vol. lxv (1909) pp. 171-72. 



