286 PROF. J. F. ELAKE OlS" THE CAMBRIAN 



of Prof. Ramsay, who says * that the beds " on the west of the por- 

 phyry are the same in number, arrangement, and lithological 

 character as those on the east." 



This point, therefore, must be carefully considered. Prof. Ramsay 

 draws a section from Dinas Dinorwic to Clegyr (fig. 55), in which 

 the two lowest members of the Cambrian undulate at low angles 

 for two miles from Dinas as far as Bryn Efail, where they are cut 

 off by a fault, and these two lowest members are also found on 

 the top of Clegyr. In spite of the undulations, however, he has 

 to make the western portion seven times as thick as the eastern 

 to make a section at all. Then we have seen that the beds 

 nearest Byrn Efail do not dip west, and that there is no fault. 

 At Dinas Dinorwic Dr. Hicks says he has found large one-inch 

 pebbles in a conglomerate. This is not mentioned by Prof. Ramsay, 

 and after twice knocking all over the hill, I failed to find it. If it 

 be there, it is followed by a tough grit, almost like a felstone, 

 which forms the mass of the hill, and must be of great thickness. 

 There is a dip of 70° inserted on the map. Then at Tyn-y-cleout 

 we have a mass of purple-banded slate, not unlike the Bangor series, 

 which appears to continue a long way. Then we reach the mass 

 of Dinas Mawr, a lofty hill composed of coarse jasper-grit, most 

 like that near Brithdir ; to the east of this we have purple slates 

 again, with interbedded grits, gently undulating for the first time, 

 but on the whole not turning up again. Such is the western 

 series (see fig. 8) ; the eastern has been described by Prof. Bonuey. 

 On the railway-section north-east of Llyn Padarn he supposes a 

 fault between thQ porphyry and the conglomerate; but there is 

 really none ; the two may be traced clinging to one another in an 

 irregular line up on to the western slope of Clegyr. On the rail- 

 way the conglomerate is not so distinct from the agglomerate as 

 above (perhaps there is no agglomerate here), and under the low 

 anticlinal further east there is only left a fissile breccia to repre- 

 sent it. After a greenstone-dyke comes a remarkable series of 

 beds, namely, banded halleflinta, as seen at Bangor, perhaps 50 feet, 

 becoming more slaty at the top, then a remarkable false-bedded band, 

 the lines being made of purple grains and the matrix felsite dust. 

 This is so compacted that it looks quite like a felsite. It is intensely 

 cleaved across the bedding and false-bedding. This is followed 

 by a one-foot band of purple-slate conglomerate pulled out into 

 lenticules, and interosculating with finer material. Then are seen for 

 100 ft. greenish slates, getting purple by degrees, and often con- 

 taining another thin band of cleaved felsite-grit, and then the purple 

 slate, which, after an interrupting fold, is seen to be the base of the 

 great roofing-slates. There is not, therefore, really much between 

 the conglomerate and the roofing-slate, and when we compare it 

 with the western beds, the series could not well be more distinct, 

 considering that they are both Cambrian. It is interesting also to 

 note that halleflinta is not peculiar to the rocks near Bangor, but is 

 rather in relation, as it appears, with a previous volcanic outburst. 

 * Mem. Geol. Surv. vol. ill. 1886, p. 154. 



