142 



WALTEE KEEPING ON THE 



of our great leader Sedgwick, published in the Society's Journal in the 

 year 1847 (vol. iii. p. 150); a slight account of the Dol Fan conglome- 

 rate, and a description of the Lampeter worm-trails in Murchison's 

 'Silurian System,' pp. 316, 317; some scattered remarks by Mr. 

 Salter and Sir Roderick Murchison in ' Siluria,' and by the former in 

 the Cambridge 'Catalogue.' The Pont Erwyd district is referred to 

 in Symonds's 'Records of the Rocks,' pp. 132, 133 ; some Graptolites 

 from Aberystwyth are recorded by Mr. J. Hopkinson, P.G.S., in the 

 ' Journal ' of the Quekett Microscopical Club, vol. i. p. 151 ; and 

 the mineral veins of Cardiganshire are described by Mr. Warington 

 Smyth, M.A., F.R.S., in the 'Memoirs of the Geological Survey,' 

 vol. ii. part 2, p. 485. 



The present communication, although worked out in a close net- 

 work of foot-routes in parts of the district, and cross traverses over 

 the rest, is offered only as a first reconnoissance report, which must 

 be followed by many labours before the structure of this great and 

 complicated district can be thoroughly mastered. Some general 

 order amidst the whirl of contortions, however, is now made clear, 

 such as the general succession of the Aberystwyth, Metalliferous, 

 and Plynlimmon groups, and the great Plynlimmon synclinal. The 

 enormous apparent thicknesses of the rocks are shown to be in part 

 due to a series of inversions ; and, further, the interpretation and 

 correlation of the beds are greatly helped forward by discoveries of 

 fossils, mostly Graptolites, in a number of localities, which afford 

 excellent data for comparison with the more distant Scotch and 

 Cumbrian Silurians. 



Even Sedgwick's work (1846) was but of a very superficial kind ; 

 for he tells us, "I profess not to know well this most contorted and 

 perplexing country." He makes four rock groups, namely the 

 (1) Aberystwyth, (2) Plynlimmon, (3) Upper South-Wales Slate 

 with the Rhyader Slate, and (4) Cambro-Silurian groups, which 

 appear in ascending order from west to east; but he adds, "the 

 sections are singularly contorted, the groups ill defined, and the 

 actual order of superposition obscure." His first group I still main- 

 tain under the name of the Aberystwyth grits ; but nearly all the 

 remaining rocks in the line of section might, I believe, have been 

 included in his second great series — the Plynlimmon group. On 

 the other hand, I have considered this great Plynlimmon group of 

 Sedgwick under two distinct headings, namely the (inferior) slate 

 series or Metalliferous-slate group, and the overlying Plynlimmon 

 grits, so that we now have the following succession of deposits : — 



(3) The Plynlimmon grits, forming a line of high country 

 in the centre of Wales, including Plynlimmon. 

 f (2) The Metalliferous-slate group, forming a broad zone of 

 Cardiganshire J contorted country on each side of (3). 



group. 1 (1) The Aberystwyth grits, best developed between Aberyst- 

 (_ wyth and Aberayron. 



Nos. (1) and (2), being closely bound together by their fossils, are known 

 together as the Cardiganshire group. 



Por an illustrative section of our area the best I can offer is that 

 from Aberystwyth, through the Devil's Bridge, to the Plynlimmon 



