528 Geological Society. 



7. Dark blue slate, of poor quality, covers the eastern flanks of 

 Arenig and Arran Mowddy, quarried at Blaen-y-cwm, where the 

 beds dip N.E. 35°, and the cleavage dips E.N.E. 55°: the lowest bed 

 of the series. 



As the Bala beds are quite unconnected with the Cambrian rocks 

 of the Berwyns, and are only overlaid by Upper Silurian deposits ; 

 as most of their organic remains are known Lower Silurian species, 

 and as the total thickness of the whole series is about the same as 

 has been assigned by Mr. Murchison to the Lower Silurians, Mr. 

 Sharpe concludes that they are the exact equivalents of the Lower 

 Silurian formation, and do not carry the series down below the beds 

 described by Mr. Murchison. Mr. Sharpe considers it as easy to 

 prove their identity with the Caradoc sandstone as with the Llandeilo 

 flags, and again endeavours to show that these must be regarded as 

 the same formation under different names. This classification re- 

 places the dark blue limestones of Bala and Coniston, on the same 

 parallel from which they were separated when Professor Sedgwick 

 adopted Mr. Marshall's view of the Silurian age of the Coniston 

 limestone, but left the Bala limestone in its erroneous position as 

 part of the Upper Cambrians. 



Mr. Sharpe adds comparative tables of the Silurian system as ex- 

 hibited in three different districts : — in Westmoi'eland, as observed 

 by himself; in Denbighshire and Merionethshire, the upper part 

 taken from Mr. Bowman's memoirs, the lower added by himself; 

 and in Shropshire, &c, as described by Mr. Murchison ; but he de- 

 fers the full comparison of these till he lays before the Society the 

 conclusion of his remarks on Westmoreland. 



Mr. Sharpe hopes that he has done away with an objection often 

 made to the Silurian system, that it wanted a definite base, and was 

 not distinctly separated from the Cambrian system ; this was not over- 

 looked by Mr. Murchison, who states that the line drawn between 

 the two systems was provisional. The difficulty arose from classing 

 with the Cambrian system many beds belonging both to the Upper 

 and the Lower Silurians, and it will vanish when this is corrected ; 

 the lower boundary of the Silurian system will then prove as distinct 

 in North Wales as in Westmoreland and Cumberland ; but to pro- 

 duce this result, the country west of Llangollen and Welsh Pool 

 must be remapped. Of the district now coloured as Upper Cambrian 

 a small share will be given to the Ludlow and Wenlock formations, 

 a larger portion to the Lower Silurians, and certain central bosses 

 of older rocks will remain for the Cambrian system : but the Upper 

 Cambrian of Professor Sedgwick, and its representative in Mr. 

 Greenough's nomenclature, the upper division of the Lower Killas, 

 must be struck out of our tables, and the Lower Silurians made to 

 rest on the true Cambrian rocks. 



The igneous rocks of Arenig and Arran Mowddy are described as 

 varying compounds of felspar and quartz. The two chains bear 

 nearly north, and their eruption is supposed by the author to have 

 modified the face of the country, and to have caused much of its pre- 

 sent complication, the prevailing strike previously having been N.N.E. 



