GEOLOGY OF CENTRAL "WALES. 



165 



probably belong here and complete the series ; for we have seen there 

 is no break of continuity down into the Bala formation. 



Part III. Palceontological Evidence. 



Although the order of succession of the forms of life have in all 

 cases been first determined from the results of stratigraphical re- 

 search, yet, that order once established, the organic remains, as 

 considered in groups, become the supreme test of the age and rela- 

 tive order of rock-masses — superseding, and in some cases actually 

 overthrowing, the apparent evidence of succession in the rock-beds. 

 Such we find to be the case over a large area of Cardiganshire, 

 where, as in the section from Aberystwyth to the Devil's Bridge, the 

 stratigraphical series, seeming to be continuously ascending through 

 some three or four miles of thickness to higher and higher beds, 

 is proved by a handful of Graptolites to be radically misleading ; 

 for, the fossils being identical at almost the extreme ends of the 

 section, the true reading is shown to be a series of inversions, by 

 which the original order of the beds is obscured. Thus, over a large 

 part of Mid Wales, it is to the organic remains alone that we can 

 look with confidence for unravelling the apparently inextricable, and 

 often delusive, tanglement of the strata, and establishing their true 

 succession. 



The most important element in our palsBontological evidence is 

 the group of Graptolites, these fossils having now become, through 

 the most successful work of Mr. C. Lapworth, F.G.S., of the highest 

 value in Cambrian and Silurian geology. All my work in this 

 branch of study is based upon the published results of that geologist ; 

 and I am also particularly indebted to him for examining many of 

 my Graptolites, as indicated in the Table (p. 170), and for valu- 

 able notes upon them conveyed to me in letters, from which I have 

 quoted below. 



A study of the Table brings out a number of important facts. 

 At the first glance it is clear that all our fossil lists exhibit 

 one and the same general geological fauna. Our richest localities 

 are Cwm Symlog (eight or nine miles east of Aberystwyth) and 

 Morben (near Machynlleth), also Cefn Hendre, Cwm, and Bryn y 

 Carnau-(in the neighbourhood of Aberystwyth). A single one of these 

 localities (Cwm Symlog) yields every species, except four, of the 

 true Graptolites known in Cardiganshire ; and this, together with 

 the intimately allied fauna of Morben Quarry, includes all our spe- 

 cies except one (Monogrcvptus turrieulatus). 



The other forms of life show no less clearly the unity of our 

 fossiliferous Cardiganshire rocks. The Ortlioceras is wide-spread 

 in the Metalliferous Slates, and occurs also amidst the Greywacke- 

 Elags of Cefn Hendre ; and the various forms of Alga3 and worm- 

 trails are also characteristic of the whole range of the Cardiganshire 

 group. 



For the typical area of reference for Graptolitic deposits we must 

 unquestionably look to the rich beds in the south of Scotland, of 

 Llandeilo to "Wenlock age, which have been so thoroughly worked 



Q. J. G. S. No. 146. x 



