224 A. Harker—Geology of Mynydd Mawr. 
of porphyry interposed a hard obstacle ; in its neighbourhood, there- 
fore, the resultant compression tended to be perpendicular to the 
boundary of this obstacle, and so the cleavage-surfaces produced 
tended to be parallel to the boundary. A little consideration will 
show that such great deviations could not be brought about by dis- 
turbance due to the intrusion of the porphyry after the impression 
of the cleavage-structure. Moreover, in such a case, the cleavage- 
planes, owing to their very high angles of dip, would have been less 
affected as regards strike than the strata. The bedding is in fact 
disturbed near the junction, but no great changes in its direction of 
strike are to be noticed. Again, we have seen that the cleavage- 
planes on the north side of the hill dip towards it, and the angle of 
dip increases on approaching the porphyry, until vertical. If the 
intrusion had disturbed the rocks after they had become cleaved, the 
reverse should be the case. 
It appears certain then that the porphyry of Mynydd Mawr is 
truly intrusive; that it is newer than the surrounding rocks, but 
older than their cleavage. The rocks in question are probably 
Arenig, though possibly older, their age, in the absence of fossils, 
being necessarily in doubt (Ramsay, Geol. N. Wales, 2nd ed. p. 168). 
The era of the cleavage is, according to all the evidence obtainable, 
the close of the Cambrian period of Sedgwick, or at least, as Sir A. 
Ramsay puts it, “‘ before the commencement of the deposition of the 
Upper Llandovery strata” (ibid. p. 326). The intrusion of Mynydd 
Mawr, then, may well be of Bala age. Ramsay remarks that the 
summit of Y Garn, about two miles ‘south of Llyn Cwellyn, which 
is mapped as “greenstone,” consists of an igneous rock similar to 
that of Mynydd Mawr, and the slates on its western side, which 
form part of the Bala stage, are seen to dip under the rock in 
question. The vertical cleavage of the slates near the junction of 
the Mynydd Mawr mass indicates that the boundary of the latter is 
nearly perpendicular. It seems very probable then that this great 
boss of porphyry is the plug of a volcanic vent of Bala age, and 
may mark the source of some of the lavas of Snowdon, Moel Hebog, 
and Llwyd Mawr. 
If this be so, a petrological examination of the Mynydd Mawr 
rock will have a special interest. Without attempting a complete 
description of it, its chief characters may be briefly noted. The 
rock is unique in appearance and easily recognized when met with 
in the drift of Moel Tryfaen and Nantlle. “To the eye it appears 
as a bluish-grey compact mass, pinkish or cream-coloured when 
weathered, in which are imbedded imperfect linear crystals of a 
lustrous black, arranged in parallel streams. The general characters 
are very constant throughout the whole extent of the hill: the black 
crystals, however, usually about a quarter of an inch in length, 
sometimes attain a larger size; felspar crystals are often visible, 
and scattered grains of quartz. The mass exhibits a platy jointing, 
sometimes in two intersecting directions, and about Craig-cwm- 
bychan the columnar structure is often very marked. 
Sir A. Ramsay (op. cit. p. 171) refers to the black crystals as 
