3li4 MISS C. A. KAI.SIN ON THK LOWKR LIMIT OJ-' 



all narrow and of uniform width (abont one inch or 1^ inch 

 wide), and should run nearly parallel. They are not, however, 

 (juite parallel to one another, nor to the line between B and C ; 

 for they undulate slightly, and even in one or two eases seem to 

 thin out. It is difficult to distinguish the rock into which they 

 have intruded from a grit ; but it seems to me more probably a 

 mass of crushed felsite, which has been brought up by faults. 

 Also the line of junction between B and C is very sharp and 

 straight, and has rather the appearance of a fault. The failure of 

 this evidence leaves as most probable Prof. Bonney's original inter- 

 pretation, that the brecciated part below should be included with 

 the conglomerate above. 



On the hillside west of the lake, I traced the conglomerate 

 beyond the " greenstone,'' and found it in close proximity to the 

 felsite ; and the conglomerate of Clegyr, according to Mr. Blake, 

 rests unconformably on the felsite. If this is the case, and if, as I 

 believe, we find no ground for considering the strata north of the 

 Llyn-Padarn felsite to be earlier than it, then the Bangor series must 

 be absent. But whether an unconformity can or cannot be in- 

 dubitably proved by any section, the distinction in the physical 

 conditions, evidenced in the rocks above and below the conglome- 

 rate, seems to justify the separation which has been made. 



(c) Physical Conditions of the Cambrian Period. 



The base of the Cambrian in most localities appears to be clearly 

 marked by a series of conglomerates with grits. In addition to the 

 widespread felsitic conglomerate, which is of so recognizable a type, 

 certain thinner layers are intercalated in many places with the 

 succeeding grits ; these may be local in their distribution, as in one 

 examjDle, on the hill near Dinas Mawr, where I found bands con- 

 sisting almost entirely of large pebbles of diabase or a basic andesite. 

 These deposits introduce the thick series of sedimentary strata. 

 According to Mr. Blake, however, the Cambrian period is to include 

 volcanic eruptions, which poured out the " mid-Cambrian " lava of 

 Llyn Padarn. There is no evidence, as far as I know, in any other 

 parts of North Wales, that the Cambrian period was one of volcanic 

 activity *. The thick deposits of fine grits and slates mark a time 

 of continuous quiet sedimentation, when the only variation was due 

 to the shallowing of the sea ; and an approach to coast -liues t. 

 Hence, we ought to require very clear evidence from this single 

 locality, if it is to be regarded as an exception to the general rule. 



* In a recent discussion on Cader Idris, it is implied that volcanic deposits 

 may occur ; but this seems onlj-a suggestion, and would apply to the time more 

 immediately preceding the Arenig, not to the * Cambrian ' of the Survey ; 

 see Quart. Journ. Geol. Soc. vol. xlv. (1889) p. 439. [Of course, if it could be 

 proved that there is no break between the sedimentary deposits of the Cambrian 

 and the underlying volcanics, as is suggested in the recent address of the Pre- 

 sident, which I have seen while this paper is passing through the press, this 

 argument would not hold.] 



t 8ee Dr. II. Hicks in Quart. Journ. Geol. Soc. vol. xxxi. (1875) p. oo^ ; 

 Prof, T. G. Eonney, ibid. vol. xxxix. (1883) p. 484, and in Eep. Brit. Assoc, for 

 1884 (Montreal Meeting), p. 543. 



