534 Trof. W. Keeping — Geology of Aherystioyth. 



Wen), clay-slate, a coarse black slate, and "Bastard" slate. Many- 

 varieties of each of these may be collected, and amongst these 

 varieties we will first notice the frequent presence in them of that 

 golden or brassy mineral Iron Sulphide. This may be found in 

 nests under Craiglais, or in cubical crystals further north towards 

 Borth, and up the valley east of Eglwys Fach, where they hand- 

 somely stud some of the grits. 



Nearly everywhere the coarser grits show some angular brown 

 spots on their weathered portions, and in some places, especially 

 at Mynydd Bach, white crystals are seen. These are, both of them, 

 crystals of felspar, or the remains of such crystals, and they are 

 interesting proofs either of the presence of older igneous rocks 

 close by, from the degradation of which they were obtained, or they 

 may even be the records of volcanos themselves then existing some 

 miles away ; for the crystals of felspar are not rounded as they would 

 be if driven for a distance by water-action, but remain still angular. 

 Microscopic analysis shows that much of the felspar belongs to the 

 plagioclase group. 



Fossils and Curious Bock Surfaces. — Wherever the crystals of iron 

 sulphide occur, we are more likely to find fossils ; at the Morben 

 Quarry, near Machynlleth, especially, we find the fossils themselves 

 converted into this bright metallic mineral. Very few of these 

 fossils have yet been recorded. Many years ago, in 1846, Professor 

 Sedgwick found fossil shells at the Devil's Bridge, which were 

 identified by Mr. Salter, and published in the Quarterly Journal 

 of the Geological Society in 1846 (Lond.), and I do not know of 

 any other list. But we need not go so far for organic remains ; 

 the quarries around Aberystwyth, at Cefn Hendre Cwm, and Bryn 

 y Mor, themselves yield those fossils called Grraptolites [Graptolites 

 Sedgivicki, G. lohiferus. Bastrites, Dictyonema, and others), long, 

 slender, toothed, saw-like organisms, each " tooth " being the cell 

 of a small " zoophyte " somewhat less highly organized than a 

 common coral. At the Morbern Quarry beautiful specimens of 

 Graptolites are abundant, including a spiral form. A species of 

 Diplograptus may be found in the Llyfnant Valley, near Glandovey ; 

 Dictyonema and Dendroid Graptolites at the Devil's Bridge ; Fora- 

 minifera [Dentalina,^ Botalia, and Textularia) occur at Cwm Symlog, 

 and shells (mostly Orthoceras) at Corris, Taren y Gesail, and 

 Steddfa Gurig. There is a curious branching calcareous organism, 

 which may perhaps be a Polyzoon, found in the quarry at Craiglais, 

 and ranging eastwards as far as Plynlimmon. 



But besides these fossils, there are other structures which will 

 force themselves upon the attention of the visitor before he sees 

 such an organism as those we have noticed above. Curious 

 markings, straight, round or irregular ; branching, curved, or con- 

 torted ; and in great variety. Some of these are undoubtedly the 

 tracks of worms ; Llampeter, for instance, is noteworthy for its 

 fine specimens of Nereites Sedgwickii and N. Cambrensis ; and 



1 This Foraminifer was first discovered by the Eev. J. F. Blake, M.A., F.G.S. 

 See Geol. Mag. Dec. II. Vol. III. p. 134. 



