1852.] SEDGWICK ON THE LOWER PAL^.OZOIC ROCKS. 137 



of the upper and lower Cambrian series *, I hesitated not to identify 

 the Coniston with the Bala limestone ; and in a short published 

 scheme f I endeavoured to bring all the rocks, from the Coniston 

 limestone to the Ireleth slates inclusive, into a provisional comparison 

 with my upper Cambrian groups ; viz. those groups which, at the 

 south end of the Berwyn chain, are superior to the Bala limestone ; 

 and are thence sent off, in great undulations, and form the physical 

 groups of a considerable portion of South Wales. This scheme I 

 now believe to have been very nearly right ; and it would have been 

 perfectly right had I not included the Ireleth slates among the equi- 

 valents of the so-called upper Cambrian system J. 



During a subsequent year (1841), on my return from Scotland, I 

 paid a very short visit to some of the Westmoreland quarries. Nearly 

 all my old collections (the accumulations of more than twenty years) 

 were at that time inaccessible to myself ; but having procured some 

 good Coniston fossils, and having received, from my friend Mr. James 

 Marshall, a still better series, they were carefully examined; and, 

 almost species by species, they agreed with the Silurian fossil lists of 

 the Caradoc sandstone. Nor was this all the evidence on which I 

 then modified my first classification §. The Coniston limestone and 

 calcareous slates appear to pass into the Coniston flagstone by almost 

 insensible gradations ; and the flagstone contains Graptolites which I 

 referred (perhaps erroneously) to the species Ludensis ; and in many 

 different places it contains whole beds of Cardiola interrupta, and a 

 few other species, which are among the characteristic lists of the 

 upper Silurian rocks. This evidence appeared at that time to be 

 irresistible ; and I so far modified my first attempt, that 1 no longer 

 brought the Coniston and Bala limestones into immediate compa-> 

 rison, but considered the Coniston limestone as the exact equivalent 

 of the Caradoc sandstone. On this hypothesis the whole series of 

 rocks (Mr. J. Otley's third great physical subdivision), from the 

 Coniston limestone upwards, formed the exact equivalents of Sir R. I. 

 Murchison's Silurian groups, from the Caradoc sandstone to the 

 upper Ludlow rocks inclusive. 



I need not detain the Society by any further reference to papers, 

 abstracts of which were published during former years ; but it was 

 obvious, from the first, that the Coniston limestone was a bad physi- 

 cal equivalent of the Caradoc sandstone ; and, on the scheme here 



* By Cambrian series was understood the whole great undulating series between 

 the Menai and the edge of Shropshire. Lower Cambrian on my first scheme in- 

 cluded all the rocks west of the Bala limestone. Upper Cambrian included the 

 Bala limestone and all the slate-rocks above it. In the present paper the Upper 

 Cambrian series (or great Bala group) is made to commence at a considerably lower 

 level ; viz. with the black slates immediately on the east side of the porphyritic 

 beds of the Great Arenig. In this way we avoid an ambiguity arising from the 

 difficulty of tracing the exact equivalent of the Bala hmestone through South 

 Wales ; and the great undulating system south of Cader Idris and east of Car- 

 digan Bay becomes at once comprehended in the Upper Cambrian series. 



t Proceedings Geol. Soc. vol. ii. p. 678. 



X It appears that the Ireleth slates are very nearly the equivalents of the Wen- 

 lock shale. 



§ Proceedings Geol. Soc. vol. iii. p. 551. 



