A. Harker—Geology of Mynydd Mawr. 223 
decreasing angles of cleavage-dip, the cleavage-structure becomes 
less perfect, and the rocks cease to be worth working for slates. 
A similar regularity may be traced in the cleavage of the rocks in 
the Cwellyn Valley on the north side of Mynydd Mawr, the average 
strike being towards N. 30° E., with small deviations, and the dip of 
the planes varying gradually as before. In the neighbourhood of 
the hill itself, however, the case is different. Considerable devia- 
tions present themselves, and these are found to follow a definite 
law: in brief, the cleavage-strike becomes more and more changed 
Sketch-Map of Mynydd Mawr District. 
A / é 
rt ral ys é 
db, AM Ane ia 
SP eae aia atin OVA 8 dN, 
By ae 2B ‘ Ww 
re Cc Wf / /Mynydd Mawr’ 
epi Pl nese 
PLO ANE cSt gh yea 
a S y, 
/ 
Scale: One inch to two miles. 
S, ‘Snowdon Ranger ;’ D, Drws-y-coed; G, Y Garn; N, Nantlle; Mc, Mynydd 
Cilewyn; Mt, Moel-y-Tryfaen: a, Archean ridge; e, Cambrian basement con- 
glomerate; 72, Slates of Nantlle quarries = Llanberis series; /, Higher slates = 
Lingula flags and newer beds ; g, Grits of Tal-y-mignedd (? Arenig age) ; 0, Slates 
of Bala stage. 
— — — Faults; ~, Dip of strata; —-+-— Strike and dip of cleavage ; 
—+— Vertical cleavage. 
as we approach the porphyry, tending always towards parallelism with 
the boundary of the intrusive mass. For instance, on the north side 
the cleavage-strike becomes deviated so as to bear always more 
easterly, as far as N. 70° HE. and even N. 86° E., with increasingly high 
dips towards the south-east and south. This variation cannot be 
traced so far as the actual junction, for the rock there is so indurated 
that it has been able to resist the mechanical stress. On the east 
side of the hill, on the other hand, the cleavage-strike is deviated 
towards the north, and in the narrow strip of slates where the lake 
approaches most closely to the porphyry, the cleavage bears due 
N.—S., with a dip of 883° to the west. Doubtless, if we could lay 
bare the whole tract, the cleavage-strike would be seen curving 
round the mass of porphyry, just as the lines of flow in a rhyolite 
curve round a porphyritic crystal, or as the fibres of wood are 
deflected by a knot. 
The only explanation of the peculiarities just described is that, 
when the whole district was operated upon by the powerful lateral 
pressure which produced the cleavage-structure, the unyielding mass 
