May, 1892 . 
A WEEK IN NORTH WALES. 
113 
described as of quartz, or agate, or jasper, none of which occur 
in the specimens we collected at the locality named. Mr. 
Harker (Bala Volcanic Series, p, 35) mentions some from near 
Pwllheli which, when broken open, present no sensible 
difference from the ordinary rhyolite. He states, however, 
that on microscopic examination a certain difference is 
perceived. The porphyritic felspars occur within as well as 
without the nodules, and the traces of fluidal lines also 
traverse them, but in addition there is “seen a faintly defined 
radial structure which can be regarded only as a giant 
skeleton splierulite.” In the specimens which I have 
examined I can see no trace of this radial structure, and in 
one, at least, there is a very marked banded structure which 
does not go beyond the border of the nodule. In one case 
where I was able to prepare a section showing the junction, 
the felsite surrounding the mass seems to flow round the 
nodule as round a solid included in the flow, so that I am in 
great doubt as to whether there is not, for this particular case, 
the explanation possible of pebbles of previously formed shaly 
rocks caught up in the lava flow. 
A walk over Snowdon from Penygwryd to Llanberis is 
not specially noticeable, except to remind geologists of the 
beautiful mass of columnar dolerite which stands up in the 
midst of the lower part of the great hollow of Snowdon, 
just by the little Llyn Teyrn. Standing just in the path of 
the great ice stream which must have filled this hollow, it is 
completely rounded and smoothed on the side of it which 
looks up the valley ; the ice has flowed right over the top of it, 
grinding it all to a perfectly curved outline, as seen from that 
direction. The face down the valley, on the other hand, is 
altogether craggy, and shows no sign of the action of ice. The 
columns are some six or seven feet high and eighteen inches 
or more thick, they are comparatively roughly shaped, and are 
more striking seen from a distance than from near at hand. 
The wild nook of Cwm Glas is accessible by an easy 
scramble from the Llanberis road up Snowdon, and is most 
thoroughly worth a visit. Whether for the desolate grandeur 
of the cliffs of Crib Goch, for the beautifully perfect evidences 
of ice action in rounding, grooving, and transporting the 
rocks, or for the magnificent profusion and luxuriance of the 
Parsley Fern and Club Moss, one visit will certainly incite 
to another. If you miss the proper place, it is a pretty steep 
scramble from the upper valley into the lower which, filled 
with moraine matter, slopes down to the Pass of Llanberis. 
At Llanberis the chief geological interest was the quartz 
felsite of what is generally considered to be the Archaean 
ridge, extending for some seven or eight miles in a N.E.-S.W. 
