XV.] CAMBRIAN AND SILURIAN IN EUROPE. 359 



rian sections is, however, due to Sedgwick and McCoy. In 

 order better to understand the present aspect of the question, 

 it will be necessary to state in a few words some of the results 

 which have been arrived at by the government surveyors in 

 their studies of the rocks in question, as set forth by Eamsay 

 in the Memoirs of the Geological Survey. In the section of 

 the Berwyns, the thin bed of about twenty feet of Bala lime- 

 stone, which (as originally described by Sedgwick) they have 

 found outcropping on both sides of the synclinal chain, is shown 

 to be intercalated in a vast thickness of Caradoc rocks ; bein^ 



* O 



overlaid by about 3,300 and underlaid by 4,500 feet of strata 

 belonging to this formation. Beneath these are 4,500 feet 

 additional of beds described as Llandeilo, which rest uncon- 

 formably upon the Lingula flags just to the west of Bala ; thus 

 making a thickness of over 12,000 feet of strata belonging to 

 the Bala group of Sedgwick. A small portion of rocks referred 

 to the Wenlock formation occupies the synclinal above men- 

 tioned. (Memoirs, III. Part III. 214, 222.) The second mem- 

 ber, in ascending order, of the Silurian system, to which the 

 name of Caradoc was given by him in 1839, was originally 

 described by Murchison under the names of the Horderley and 

 May Hill sandstone. The higher portions of the Caradoc were 

 subsequently distinguished by the government surveyors as 

 the Lower and Upper Llandovery rocks ; the latter (constitut- 

 ing the May Hill sandstone, and known also as the Pentamerus 

 beds), being by them regarded as the summit of the Caradoc 

 formation. In 1852, however, Sedgwick and McCoy showed 

 from its fauna that the May Hill sandstone belongs rather to 

 the overlying Wenlock than to the Caradoc formation, and 

 marks a distinct palseontological horizon. 



This discovery led the geological surveyors to re-examine the 

 Silurian sections, when it was found by Aveline that there 

 exists in Shropshire a complete and visible want of conformity 

 between the underlying formations and the May Hill sand- 

 stone ; the latter in some places resting upon the nearly verti- 

 cal Longmynd rocks, and in others upon the Llandeilo flags, 

 the Caradoc proper or Bala group, and the Lower Llandovery 



