No. 3.] HUNT — ON CAMBRIAN AND SILURIAN. 28^ 



wick's or in Murcliison's sections was evident, and the Government 

 surveyors, while sustaining the correctness of those of Sedgwick, 

 have shown the sections of Murchison to have been completely 

 erroneous. 



The first step towards an exposure of the errors of the Silurian 

 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 Ramsay in the Memoirs 

 of the Geological Survey. In the section of the Berwyns, 

 the thin bed of about twenty feet of Bala limestone, 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 ; being 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 de- 

 scribed as Llandeilo, which rest unconformably 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 oc- 

 cupies the synclinal above mentioned. [Memoirs, III, part 2, 

 214, 222.] The second member, 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 31 ay Hill sandstone. The higher portions of the 

 Caradoc were subsequently distinguished by the Government sur- 

 veyors as the Lower and Upper Llandovery rocks ; the latter (con- 

 stituting the May Hill sandstone, and known also as the Penta- 

 merus 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 paleontological 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 sandstone ; the latter 

 in some places resting upon the nearly vertical Longmynd rocks, 

 and in others upon the Llandeilo flags, the Caradoc proper 

 or Bala group, and the Lower Llandovery beds. Again, in 

 Vol. VI. L No. 3, 



