XV.] CAMBRIAN AND SILURIAN IN EUROPE. 353 



Lower Cambrian, included the crystalline rocks of the south- 

 west coast of Caernarvon and a considerable portion of Angle- 

 sea, and consisted of chloritic and micaceous schists, with slaty 

 quartzites and subordinate beds of serpentine and granular 

 limestone ; the whole without organic remains. 



These crystalline rocks were, however, soon afterwards ex- 

 cluded by him from the Cambrian series, for in 1838 (Proc. 

 Geol. Soc., II. 679) Sedgwick describes further the section 

 from the Menai Strait to the Berwyns, and assigns to the 

 chloritic and micaceous schists of Anglesea and Caernarvon a 

 position inferior to the Cambrian, which he divides into two 

 parts ; namely, Lower Cambrian, comprehending the old slate 

 series, up to the Bala limestone beds ; and Upper Cambrian, 

 including the Bala beds, and the strata above them in the Ber- 

 wyn chain, to which he gave the name of the Bala group. 

 The dividing line between the two portions was subsequently 

 extended downwards by Sedgwick to the summit of the Arenig 

 slates and porphyries. The lower division was afterwards sub- 

 divided by him into the Bangor group (to which the name of 

 Lower Cambrian was henceforth to be restricted), including the 

 Llanberris roofing-slates and the Harlech grits or Barmouth 

 sandstones ; and the Festiniog group, which included the Lin- 

 gula flags and the succeeding Tremadoc slates. 



In the communication of Murchison to the same Dublin 

 meeting, in August, 1835, he repeated the description of the 

 four formations to which he had just given the name of Si- 

 lurian ; which were, in descending order, Ludlow and Wen- 

 lock (Upper Silurian), and Caradoc and Llandeilo (Lower Si- 

 lurian). The latter formation was then declared by Murchison 

 to constitute the base of the Silurian system, and to offer in 

 many places in South Wales distinct passages to the underly- 

 ing slaty rocks, which latter were, according to him, the Upper 

 Cambrian of Sedgwick. 



Meanwhile, to go back to 1834, we find that after Murchi- 

 son had, in his communication to the Geological Society, de- 

 fined the relation of his Llandeilo formation to the underlying 

 slaty series, but before the names of Silurian and Cambrian 



