GEOLOGY OF CENTRAL WALES. 



147 



flags may be readily extracted, from which feature I have called this 

 zone the " Large-flag series" (fig. 2). The dip is to the west, running 

 under the slaty series. Fossils may be found here in the open quarry 

 above the Devil's Punch-bowl, and along the sides of the waterfall 

 itself; but all the Dendroid Graptolites (Cladophora) are from a 

 band above the iron bridge in the waterfall-grounds ; while some 

 other beds below this bridge are so crowded with Crinoid ossicles as 

 almost to merit the name of Limestone. 

 Fossils from the Devil's Bridge : — 



Odontocaulis Keepingii, Lapw. 



(n. sp.). 

 Chonetes leevigatus, Sby. ? 

 Orthis, sp. 



Other fragmentary Brackiopods. 

 Phacops, n. sp. 

 Fragments of Encrmites. 

 Nereites Sedgwickii, Murch. 

 Myrianites tenuis, M" Coy. 



Monograptus Sedgewickii, Portl.? 



spiralis, Geinitz. 



turriculatus, Barr. 



Dictyonema venustum, Lapw. (n. sp.). 



corrugatellum, Lapw. (n. sp.). 



Calyptograpsus (?) plumosus, Lapw. 

 (n. sp.). 



Rhizograptus (?) digitatus, Lapw. 



(n. sp.). 

 ramosus, Lapw. (n. sp.). 



Now in this list of fossils the species of Graptolites are seen to 

 correspond with those of Cefn Hendre, the occurrence of Mono- 

 graptus turriculatus in both places being an especially important 

 fact, this being a species of very limited range. 



And we have seen that they occur in identical rocks in the two 

 places ; I therefore cannot doubt that these are really one and the 

 same set of beds, seen in the upper part of the Aberystwyth grits at 

 Cefn Hendre and reappearing at the surface in an anticlinal at the 

 Devil's Bridge. 



This conclusion, however, is in direct antagonism with the strati- 

 graphical appearances ; for, notwithstanding the numerous folds, the 

 predominance of the easterly dip is most determined and impresses 

 itself strongly upon the mind. 



Being convinced of these appearances, it was determined to test 

 the thickness of the beds by actual measurement; and our exact 

 observations and calculations, made at more than 100 exposures 

 along the Devil's-Bridge road from Aberystwyth, strikingly con- 

 firmed our earlier impressions, giving, indeed, a result of nearly four 

 miles thickness of strata (3 miles 1612 yards). In such apparent 

 conflict of evidence, and in the absence of large faults, the only 

 reasonable explanation is that the original natural order of the 

 rocks has been destroyed by the formation of a great inversion, or 

 rather, as I believe, a series of inversion folds in the Metalliferous- 

 slate series (see fig. 1). And indeed this interpretation, in con- 

 formity with the fossil evidence, is independently almost demanded 

 to explain away the enormous apparent thicknesses of similar rock- 

 beds as measured from their present dips. 



"We may then safely conclude that in the Devil's-Bridge rocks we 

 are again upon the upper part of the Aberystwyth-grit series. 



Continuing our section eastward over the hill through the Arch 

 to Cwm Ystwyth (fig. 2), we still traverse a series of shaly slates of 

 the " Metalliferous " type, with here and there thin grits apparently 



