Vol. 65.] SUCCESSION ABOUND PLYNL1M0N AND PONT EKWYD. 467 



In the Geological Magazine for 1906 (8) I gave a very brief 

 sketch of the succession and of the structure of the area, so far as 

 it had been investigated at that time. 



A few species of graptolites from the Pont Erwyd district were 

 figured by MissElles & Miss Wood in the Monographs of the Palae- 

 ontographical Society, vol. lxi, issued for 1907 (9). 



Literature. 



(1) 1847. A. Sedgwick. ' On the Classification of the Fossiliferous Slates of 



North Wales, &e.' Quart. Journ. Geol. Soc. vol. iii, p. 133. 



(2) 1848. W. W. Smyth. ' On the Mining District of Cardiganshire & 



Montgomeryshire ' Mem. Geol. Surv. vol. ii, pt. 2, p. 655. 



(3) 1866. A. C. Ramsay. ' The Geologv of North Wales ' Mem. Geol. Surv. 



vol. iii, 1st ed. 2nd ed. 1881. 



(4) 1869. J. Hopkinson. 'On British Graptolites' Journ. Quekett Microscop. 



Club, vol. i (1869) p. 151 & pi. viii, figs. 4a-4 b. 



(5) 1881. W. Keeping. ' The Geology of Central Wales, with an Appendix 



by C. Lapworth ' Quart. Journ. Geol. Soc. vol. xxxvii, p. 141. 



(6) 1900. H. Lapworth. ' The Silurian Sequence of Rhayader ' Quart. Journ. 



Geol. Soc. vol. lvi, p. 67. 



(7) 1906. H. Lapworth. ' The Geology of Central Wales' Froc. Geol. Assoc. 



vol. xix (1905-1906) pp. 160 & 232-33. 



(8) 1906. O. T. Jones. 'The Geology of the Plynlimmon District' Geol. Mag. 



dec. 5, vol. iii, p. 336. 



(9) 1907. G. L. Elles & E. M. R. Wood. ' Monograph of British Grapto- 



lites,' pt, vi, pp. 258, 264, 265 & text-figs. 181 a-c, p. 266 &pl. xxxi, 

 figs, lie, 12 b, c, d, 14 b, c. 



III. The Stratigeaphical Sttccession. 



The great series of shales, mudstones, and grits reaching a 

 thickness of between 7000 and 8000 feet which occurs in the Pont 

 Erwyd district has been divided into three primary divisions or 

 Stages, which have been further divided into Groups ; and in most 

 cases it has been possible to recognize, within the latter, minor sub- 

 divisions generally characterized by a certain species or assemblage 

 of species of graptolites, when they are termed zones, one of the 

 species being chosen as an index of the zone. Occasionally a still 

 smaller subdivision into subzones or ' bands ' has been adopted, 

 which may only apply locally in some cases, but in others can be 

 shown to hold over wide areas outside the district. It has been 

 thought desirable to give local names to the Stages and Groups, and 

 although this tends to the introduction of a considerable number of 

 such names into geological literature, it is considered safer to do 

 so than to correlate the formations directly with those of distant 

 areas, a process which is always liable to some error, and one 

 into which the personal element enters of necessity. The general 

 classification used throughout this paper is, therefore, as follows 

 (in descending order) : — 



