25 i EEV. J. F. BLAKE ON THE EOCKS MAPPED AS 



dips, however, are so irregular, and tlie variations of one grit might 

 so easily he extended to include those of the other, that it is only the 

 most probahle supposition, from various stratigraphical considerations 

 too numerous to mention, that the grit next the pale slate is really 

 Khiw-wen Grit in its proper place. On the opposite side of the 

 Ogwen Yalley fault (vrhere the beds run in circles), though the beds 

 below the Purple Slate are often gritty near the top, no distinct 

 band can be made out, especially as the clearest line of junction is 

 certainly faulted, and it is the same at Bangor. The ' Halle- 

 flinta ' is there imimediately followed by the gritty Purple Slate. 



On the south side of Llyn Pa'larn there is a green grit occu- 

 pying this position, west of the Glyn Peris Hotel, that is to say, 

 it is seen to rise from beneath the Purple Slate in the quarries and 

 is seen also to overlie Pale Eanded Slate in the cliff at the bend of the 

 Cwm-y-glo road. There is also a grit of similar character in a 

 corresponding position on the opposite or northern side of the lake. 

 Hereabouts, however, there is a complication in relation to the 

 felsite, so that some of the rocks in this position have a very 

 remarkable character, the description of which must be left till the 

 felsites are dealt with. 



On the south side of the Pothell, at Pontrhythallt, a reddish grit 

 crosses the stream, causing rapids, and this occupies the position 

 of the Ehiw-wen Grit — if the beds above are true Purple Slate, 

 but nothing more is seen of any such rock to the south of 

 this spot. 



§ 6. The Pale Bakded Slates and Halleflintas. 



This is a very important group of rocks, requiring to be carefully 

 distinguished from the Purple Slates. Here and there perhaps the 

 Banded Slates may put on a slightly purple hue, but this is very 

 rare. The Purple Slates are characteristically massive, while these, 

 especially when most slaty or grittj', have invariably a well-marked 

 banded structure. When they put on the ' halleflintoid ' form these 

 bands reduce to fine laminae, which, though often much contorted, 

 are still recognizable. The series is well seen over all the ground 

 that lies to the north of the felsite masses. Thus on Moel Bhiw-wen 

 (see fig. 5, p. 250) hard Pale Banded Slates occupy the western half 

 of the summit and all the western slopes, where they bend over 

 slightly, so as to have a small westerly dip, as is roughly shown in 

 Sir A. Ramsay's section. We here get a good idea of their importance, 

 for they occupy ground differing in elevation by 600 feet from the 

 top to the bottom, in which the eastern and western dips may 

 nearly compensate each other. On Moel-y-Ci (see fig.. 6, p. 253), with 

 the exception of the summit-capping of grit and the conglomerates on 

 the west, the Pale Banded Slates occupy the whole of the ground 

 from a height of 1344 feet down to about 500 feet, with very little 

 on the whole to be allowed for dip. North of this they occupy a 

 broad band of country, throughout which they have an easterly dip, 



