78 E. Wynne Hughes — Geology of 



conglomerate. Both the conglomerate and the quartz grit dip 

 steeply to the south-east. A. further point of interest is the 

 existence of several well-marked faults, running north-east to 

 south-west. These faults have the same direction as the fault 

 between the grits and the Ordovician slates on the south-east side 

 of Cil-y-Coed. 



Several thin sections from the conglomerate in the adit were 

 examined ; they show that the conglomerate bears a close resemblance 

 to that at Craig- j' -Din as and Cil-y-Coed. Thin sections of the grit 

 from the two localities are also very similar, though perhaps the 

 Moel Tryfan grit approximates more closely to a true quartzite. 



The adit shows that the whole mass has undergone great dis- 

 turbance, there being at least six faults visible in the adit, with the 

 result that the structure is highly complicated. The conglomerate, 

 for instance, is only 12 feet thick in the adit, whereas on the moun- 

 tain top it has an outcrop fully 120 feet wide. Between the 

 conglomerate and the porphyry at the west end of the adit, is 

 a thickness of fully 200 yards of green slate, grit, gritty slate, and 

 a green chloritic rock. Whether these slates and grits are Lower 

 Cambrian or Pre-Cambrian is a question of some difficulty which 

 I hope to investigate at a future date. t 



An examination of the adit then shows that — 



(a) The conglomerate and quartz grit here bear a close resemblance to those 



at Cil-y-Coed and Craig-y-Dinas. 



(b) They are in the same relative position. 



(c) Their dip, although steeper, is in the same direction. / 



(d) As at Craig-y-Dinas, they are overlain by purple slate. 



(e) The conglomerate in the adit does not lie directly on the quartz-porphyry. 

 (/) The matrix of the conglomerate is more argillaceous than that at Craig- 

 y-Dinas and Cil-y-Coed. 



B. Bwlch-y-llyn and Cilgwyn. — The conglomerate was traced in 

 a south-westerly direction. At Bwlch-y-llyn the quartz grit also 

 comes to the surface, and is again lying directly upon the con- 

 glomerate on its south-east side. An examination of microscopic 

 sections cut from specimens in this locality again brings out a close 

 resemblance between the grit here and that at Cil-y-Coed and 

 Craig-y-Dinas. Like the latter, the grit at Bwlch-y-llyn ranges 

 from fine to coarse, becoming more quartzitic from north-west to 

 south-east. (PI. II, Figs. 4, 6.) 



From this point on, the conglomerate forms the high ground on 

 Mynydd Cilgwyn, and it persists in a south-westerly direction along 

 the whole length of the mountain. On the south-west slopes of the 

 mountain, however, it disappears in the manner indicated in the 

 Survey memoir. The pebbles in it, on this portion of the ridge, are 

 certainly less numerous, but they are decidedly larger than at Moel 

 Tryfan, and, what is still more interesting, the matrix is distinctly 

 more felspathic. In hand specimens and under the microscope this 

 gritty matrix could hardly be distinguished from the gritty matrix 

 of the conglomerate at Cil-y-Coed. (PI. II, Figs. 1, 2.) 



Owing to the presence of several quarries on the east side of the 

 ridge, the purple slate can easily he traced in a S.S.E. direction 

 from Moel Tryfan to Cilgwyn. At Cilgwyn it is exposed at the 



