222 A. Harker—Geology of Mynydd Mawr. 
through a regular succession of Cambrian (or Ordovician) rocks, 
an examination of the junction affords convincing proof of its 
intrusive character. On the northern flanks of the hill the relations 
are well exhibited, and the induration and extensive mineralogical 
alteration of the slate in the vicinity of the boss are very marked. 
It is indeed by no means easy to determine on the ground the precise 
point where the hardened, semicrystalline slate gives place to the 
porphyry. 
The question may, however, be approached on a different line, 
which indicates a higher, as well as a lower, limit to the age of the 
intrusion. <A series of observations with compass and clinometer 
may, I believe, be made to yield interesting results in many districts 
of cleaved rocks, and so no apology seems needed for introducing 
in this connection a few remarks on the arrangement of the cleavage- 
planes in the slaty rocks of the Vale of Nantlle. 
All the peculiarities of slaty cleavage may be studied to advantage 
in the slate-quarries of the lower part of the valley, which are 
worked in Lower Cambrian rocks resembling those of Llanberis. 
Here, as usual, a great perfection of the cleavage-structure is 
found associated with high angles of cleavage-dip. The cleavage- 
strike seldom deviates more than a few degrees from N. 36° H.— 
S. 36° W., which is also about the normal strike of the strata, in 
accordance with the law developed by Sedgwick, Darwin, Jukes, 
and others. It also agrees nearly with the trend of the great 
Archean ridge, which extends from Llanllyfni to beyond Llanberis. 
The quartz-porphyry composing this ridge has itself been structurally 
affected by the same stress which originated the cleavage of the 
neighbouring slates. The platy structure of the mass, well seen 
at Cilgwyn, Pen-y-groes, etc., has the same strike; it is not mere 
jointing, but is often seen to be in direct relation with a certain 
schistosity of the quartz-porphyry. 
The variations of cleavage-dip in the Nantlle slates agree well 
with the rules laid down by Sharpe forty years ago. <A little west 
of the lower lake runs a line of vertical cleavage, nearly coinciding 
with the bye-road from Tal-y-sarn to Llanllyfni. On each side of 
this line of strike the cleavage-planes dip away from it at high 
angles, so that the cleavage-dip in the Pen-y-bryn and other quarries 
near Nantlle itself (Nant-y-llef on the maps) is south-easterly. 
Another line of vertical cleavage passes through the middle of the 
upper lake, and in the slates on each side of this the cleavage-planes 
dip in towards this line. The north-westerly cleavage-dips hold up 
the valley with a gradually decreasing angle, to 56° at Drws-y-coed 
Pass. The strike of the cleavage remains very constant in these 
higher beds, although the strata themselves in the neighbourhood of 
the Tal-y-mignedd copper-lodes change their bearing considerably, 
curving round towards the east. This is an example of the law that 
the cleavage of a district, while following the trend of the main 
axes of disturbance, is independent of minor variations in the strike 
of the strata. Occasionally local irregularities occur, which are 
clearly traceable to subsequent faulting. It is noticeable that, with 
