Oh. XXXILT CAMBEIAN VOLCANIC EOCKS. 559 



rieiies , and the interposed Llandeilo flags are of sandstone and sliale, 

 with trilobites and graptoHtes.* 



Cambrian Volcanic Bocks. — In a former chapter (Ch. XXVII. p. 447), 

 we have seen that below the Llandeilo and Bala beds of Lower Silurian 

 date there occur, in North Wales, a series of rocks of vast thickness, 

 which may be called Cambrian. The upper subdivision, named by Pro- 

 fessor Sedgwick the " Festiniog group," comprises, first, the Arenig Slates, 

 YOOO feet thick in North Wales, in the midst of which dense masses of 

 porphyry, trap-conglomerate, and other igneous rocks, which are supposed 

 by Professor Sedgwick to be of contemporaneous origin, are intercalated ; 

 secondly, the Lingula flags underlying the former, and of which the fossils 

 were treated of at p. 448 ; thirdly, still lower, the Bangor group or.Lower 

 Cambrian, in which bands of felspathic porphyry occur. These last are, 

 in the opinion of Professor Ramsay, intrusive and not of the same date as 

 the associated sedimentary deposits. 



Professor Sedgwick has also described, in his account of the geology of 

 Cumberland, various trap rocks which accompany green slates, agreeing 

 in mineral character and aspect with the Arenig Slates, which underlie 

 all the fossiliferous strata of Cumberland, and consist of felspathic and 

 porphyritic rocks and greenstones, occurring not only in dikes, but in 

 conformable beds. Occasionally there is a passage from these igneous 

 rocks to some of the green quartzose slates. These porphyries are sup- 

 posed to have been produced contemporaneously with the stratified chlo- 

 ritic slates by submarine eruptions oftentimes repeated, the materials of the 

 slates having been supplied, in part at least, from the same source.f 



* Murchison, Siluvian System, &c. p. 325. 

 f GeoL Trans. 2d series, vol. iv. p. 65. 



