XV.] CAMBRIAN AND SILURIAN IN EUROPE. 351 



until at length the eastern dip of the strata is exchanged for a 

 westward one, thus giving to the Berwyn chain, like that of 

 Snowdon, a synclinal structure. As a consequence of this, the 

 limestone of Bala reappears on the eastern side of the Berwyns, 

 underlaid as before by a descending series of slates and por- 

 phyries. These results, with sections, were brought before the 

 British Association for the Advancement of Science at its 

 meeting at Oxford, in 1832, but only a brief and imperfect 

 account of the communication of Sedgwick on this occasion 

 appears in the Proceedings of the Association. He did not at 

 this time give any distinctive name to the series of rocks in 

 question. (L. E. & D. Philos. Mag. [1854] (4), VIII. 495.) 



Meanwhile, in the same year, 1831, Murchison began the 

 examination of the rocks on the river Wye, along the southern 

 border of Radnorshire. In the next four years he extended 

 his researches through this and the adjoining counties of Here- 

 ford and Salop, distinguishing in this region four separate 

 geological formations, each characterized by peculiar fossils. 

 These formations were, moreover, traced by him to the south- 

 westward, across the counties of Brecon and Caermarthen; 

 thus forming a belt of fossiliferous rocks stretching from near 

 Shrewsbury to the mouth of the river Towey, a distance of 

 about one hundred miles along the northwest border of the 

 great Old Red sandstone formation, as it was then called, of 

 the west of England. 



The results of his labors among the rocks of this region for 

 the first three years were set forth by Murchison in two papers 

 presented by him to the Geological Society of London in Janu- 

 ary, 1834. (Proc. Geol. Soc., II. 11.) The formations were 

 then named as follows in descending order : 1. Ludlow, 2. 

 Wenlock ; constituting together an upper group ; 3. Caradoc, 

 4. Llandeilo (or Builth) ; forming a lower group. The Llan- 

 deilo formation, according to him, was underlaid by what he 

 called the Longmynd and Gwastaden rocks. The non-fossilif- 

 erous strata of the Longmynd hills in Shropshire were described 

 as rising up to the east from beneath the Llandeilo rocks ; and as 

 appearing again in South Wales at the same geological horizon, 



