CAMBRIAN IX CAEENARVOXSHIEE. 255 



from the high ground at Sling down to the crossing of the riA^er near 

 the Aber fault. In this region they appear to be confounded on the 

 Geological Survey map with the Bronlhvyd Grit, from which they 

 are absolutely distinct. They also occupy a large area between 

 Tregarth and the Eiver Ogwen, where they also in parts have an 

 easterly dip, as is well seen in the mineral-railway cutting and in 

 the crags above it, but this area is supposed to be cut off from the 

 last by a fault (see fig. 7, p. 253). They are also brought up in a 

 wedge at Pare Bryniau, where they have a northerly dip. On the 

 east side of the River Ogwen they occuj)y a semicircular area, where 

 they are seen b}' their dips to form one half of a dome, of which we 

 do not see the base, the lowest beds exposed being the great con- 

 glomerate to be presently mentioned. 



It is these same rocks, of a somewhat more halleiiintoid character 

 than usual, yet always either banded or laminated, which occupy 

 the heights of Bryniau Bangor, extending from Minffordd northward 

 to the end of the Recreation Ground. 



When we have gained some familiarity with the rocks of this 

 portion of the series in those localities w^beie they are characteristi- 

 cally developed and where there can be no doubt of their position, 

 we are better fitted to examine them in the areas where more doubt 

 may be felt or has been expressed. One of these is the country 

 north-west of Bryn Efail to beyond Dinas Mawr. In several spots 

 at Llys Dinorwig and Bryn Derw, and in the cutting near Coed 

 jSIadog figured by Miss Raisin,^ exposures may be seen. Miss Raisin 

 calls the rocks ' argillites,' a term which fairly expresses their 

 general character. "I'here is, however, some grit amongst them ; 

 but the main point is that they are essentially banded and grey, and 

 of one thing we may be certain, that no such rocks are met with 

 above the Rhiw-wen Grit. The section in the cutting is very 

 instructive in relation to controverted points. The rocks are more 

 or less contorted, as in many other places, and the result is that they 

 dip in different directions in diff'erent parts. This accounts for our 

 finding other spots where the dip is reversed ; but as, on the whole, 

 the beds descend towards the east in the cutting, so the majority of 

 the dips (in fact all but one) are to the east in the isolated expo- 

 sures. At Bryn Derw there is a small patch of an earthy conglo- 

 merate, in which felsite-pebbles must be very scarce, if any are 

 present, for I could find none. There is also a crag of grit, and by 

 the side of the main road to the west is a boss of purple-banded 

 grit. In the light of later knowledge I am not prepared to say what 

 these are ; if they belong to the series they may represent the 

 bottom of the Rhiw-wen Grit. 



Beyond Dinas Mawr the rocks, though banded, are more purple 

 and gritty than in any other area of rocks referred to the same 

 portion of the series, and in one place there appears to be some 

 felsite. Yet the base of Dinas Mawr itself is fairly characteristic 

 hard Pale Banded Slate, with a dip of 50° or more to the east. In 



^ Quart. Journ. Geol. Soc. vol. xlvii. (1891) p. 335. 



