Trof. T. Sterry Hunt — On Cambrian and Silurian. 387 



in the study of these rocks, Murchison, as he tells us, ui'ged him to 

 give them a British geographical name. Sedgwick accordingly pro- 

 posed for this great series of Welsh rocks the appropriate desig- 

 nation of Cambrian, which was at once adopted by Murchison for 

 the strata supposed by him to underlie his Silurian system. (Mur- 

 chison, Anniv, Address, 1842; Proc. Greol. Soc, vol. iii. p. 641 ) 

 This was almost simultaneous with the giving of the name of 

 Silurian, for in August, 1835, Sedgwick and Murchison made com- 

 munications to the British Association at Dublin on Cambrian and 

 Silurian Eocks. These, in the volume of Proceedings (pp. 59, 60), 

 appear as a joint papei', though from the text they would seem to have 

 been separate. Sedgwick then described the Cambrian rocks of North 

 Wales as including three divisions : 1. The Up]Der Cambrian, which 

 occupies the greater part of the chain of the Berwyns, where, ac- 

 cording to him, it was connected with the Llandeilo formation 

 of the Silurian. To the next lower division Sedgwick gave the 

 name of Middle Cambrian, making up all the higher mountains of 

 Caernarvon and Merionethshire, and including the roofing-slates and 

 flagstones of this region. This middle group, according to him, 

 afforded a few organic remains, as at the top of Snowdon. The 

 inferior division, designated as Lower Cambrian, included the crystal- 

 line rocks of the south-west coast of Caernarvon and a considerable 

 portion of Angiesea, 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 excluded 

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

 vol. ii. p. 679) Sedgwick describes further the section from the Menai 

 Strait to the Berwyns, and assigns to the chloritic and micaceous 

 schists- of Angiesea and Caernarvon a position inferior to the 

 Cambrian, which he divides into two parts, viz. 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 Berwyn chain, to which he gave the name of the Bala 

 group. The dividing line between the two portions was subse- 

 quently extended downwards by Sedgwick to the summit oF the 

 Arenig slates and porphyries. The lower division was afteiwards 

 subdivided 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 ssnnd- 

 stones ; and the Ffestiniog group, which included the Linguhi-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 Silurian; which were, in de- 

 scending order, Ludlow and Wenlock (Upper Silurian), and Caradoo 

 and Llandeilo (Lower Silurian). 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 

 underlying slaty rocks, which were, according to him, the Upper 

 Cambrian of Sedgwick. 



