332 MISS C. A. RAISIN ON THE LOWER LIMIT OF 



rocks. Further, thoy must be supposed to date from pre-Gambrian 

 times, since the granitoid which intrudes into them is so classed 

 on the map; so that wo should here have Arenig strata resting 

 upon pre-Cambrian rocks. In the Survey Memoir*, Sir A. 

 Ramsay gives the evidence for his view, that the unconformity at 

 Caernarvon has cut out in the space of four miles the Limjula 

 beds of Elidyr Fawr, which are described as being about 

 2000 feet in thickness. Mr. Blake's hypothesis demands much 

 more than this — that the beds which are wanting include " all 

 Cambrian rocks." These, according to the Survey, would 

 be the Limjida beds above mentioned, the " Cambrian grits " 

 given as 1700 feet thick t, and the "Lowest Cambrian" of the 

 Survey Memoir. But, according to Mr. Blake's theory, below 

 these " Lowest Cambrian " of the Survey, we have to include in 

 the same period the old rhyolite of Llyn Padarn, the Lower Cam- 

 brian (of Mr. Blake) of Dinas Mawr, &c., and — either as equivalent 

 to these last or below them — the Bangor beds. If we assume this 

 to be the true succession, a very extended series of strata is wanting 

 at Caernarvon ; and also the thickness and importance of the Cam- 

 brian formation would be much increased. 



III. The Age of the Southern Eelsite. 



The new theory must then find its chief support in the interpre- 

 tation of the southern felsile and the rocks associated with it. 

 Here Mr. Blake agrees that the felsite was a lava older than the 

 slates and grits to the south, but he argues that the lava flowed over 

 the sedimentary strata now exposed to the north of it. This 

 view is supported by two arguments — the one being the lithological 

 character of the beds to the north of the Llyn-Padarn felsite ; the 

 other, the evidence of the quarry section at Bryn Efail. 



(a) Lithological Comparison of the Sedimentary Strata. 



As regards the former, certain of the rocks are said to resemble 

 strata in the Bangor district. It would be quite possible that some 

 of the pre-Cambrian series might occur over this area, cropping out 

 under the lower beds of the Cambrian, although I have not found 

 any rocks which I could refer with certainty to the older formation, 

 Mr. Blake, however, contrasts the strata north and south of the 

 Llyn-Padarn felsite, and states that " the [two] series could not 

 well be more distinct, considering that they are both Cam- 

 brian " %. But the argillites near the mineral railway along 

 the lake, and those north of Bryn Efail and by Dinas Mawr^ 

 are certainly in many places indistinguishable from each other, 

 while the grits of Dinas Mawr and Bryn Madog could be matched 

 by some of those on the hillside west of the lake, or on Clegyr. It 

 is true that the workable slates are not exposed to the north, but 

 they may possibly be concealed beneath the extensive deposits- 



* ' Geol. of North Wales,' 2nd ed. p. 252. 



t Ibid. p. 1G6. 



X Quart. Journ. Oeol. Soc. vol. xliv. (1888) p. 286. 



