part 2] ITS STEUCTUEE ANJ) EOCK- SUCCESSION. 133 



early as 1832, This paper was later incorporated into a more 

 general treatise. i As he did not full}^ realize the complexities of 

 the structure of the country, Sedgwick's succession has naturally 

 undergone some modification in later days, but the papers are 

 notevvorthy since he clearly understood that the faunas of the 

 Rhiwlas and Bala Limestones were distinct ; he drew attention, 

 moreover, to the nature of the beds above the Bala Limestone, 

 and indicated the true position and character of the Hirnant 

 Limestone. 



J. de C. Sowerby & J. W. Salter, who contributed notes on the 

 fossils, remarked on the paucity of brachiopods in the Ehiwlas 

 Limestone as compared with their abundance in the Bala Lime- 

 stone, mention being made of Ortliis acionicB as a highly charac- 

 teristic form. They also record the distinctive peculiarities of the 

 brachiopods of the Hirnant Limestone. 



In the paper published in 1852 2 on the ' Classificiition & Nomen- 

 clature of the Lower Palaeozoic Rocks of England & Wales,' 

 Sedgwick notes the Coniston Limestone as the equivalent of the 

 Bala Limestone, and subdivides the Bala Group of rocks into an 

 Upper and Lower, the Upper Bala beginning with the Bala Lime- 

 stone, including the Hirnant Limestone and Shelly Sandstone, and 

 ending with the dark indurated Shales, passing in places into a bad 

 pyritous roofing-slate. The position of the Bhiwlas Limestone is 

 not indicated in this paper. 



The area was officially surveyed by J. B. Jukes, and the results 

 of his work are given in the two editions of the Geological Survey 

 Memoir on North Wales. ^ It is marvellous that Jukes was able 

 to show so much on the 1-inch map, and it is clear from his notes 

 that his general conclusions accorded more completely with those 

 that I have reached than the 1-inch map would indicate, although 

 my own conclusions have only been arrived at after laborious 

 mapping on the 6-inch, and often on a still larger scale. Thus, in 

 the end, Jukes was convinced that there was only one ash-bed 

 in the lower part of the series east of Bala Lake, although he notes 

 the possibility of the existence of more elsewhere, and records the 

 occurrence in places of a definite ash-bed immediately below the 

 limestone. My work has confirmed the existence of only one ash 

 in the lower part of the series, not merely in the area east of 

 the lake, but on the west side also, and again in the ground on 

 the north so far as I have seen it. Jukes is also perfectly definite 

 as to the nature of the Bala Limestone, and, from his remarks 

 concerning the ash immediately below the limestone, it seems 

 evident that he included in the Bala Limestone not only the massive 

 limestones, but the whole of the Calcareous Ash — in fact, he 



1 Q. J. G. S. vol. i (1 845) p. 6. 

 ^ Ihid. vol. viii, p. 136. 



3 Mem. Geol. Surv. vol. iii (1866) chaps, xiii & xiv; vol. iii, 2nd. ed. (1881) 

 chaps. XV & xvi. 



