170 
THE GEOLOGIST. 
Bwlch-drws-ar-Dudwy, the bold outlines of the Arenigs, the Moelwyns, 
and Cynicht, all meet the eye almost at a glance ; and, far iu 
the distance, the peak of Snowdon sharply pierces the air, rising 
to a height of 3,571 feet above the sea. The reason of the 
extreme diversity of the scenery from Cader Idris northwards is, 
that the greater proportion of that district of Wales is formed of nume- 
rous interstratifications of slate and felspathic traps and ashes, 
mingled with large intrusive felspathic bosses and lines of greenstone. 
The whole of these have been disturbed, and thrown into numerous 
sweeping anticlinal and synclinal curves ; and, all being alike subject to 
denudation, portions of the softer slaty beds have been more rapidly 
worn away ; while, in great part resisting denudation, the igneous rocks 
and some of the grits stand out in bold relief, forming great part of the 
cliffs of Cadir Idris, the Arans, the Arenigs, the Moelwyns, Y-Glyder- 
fawr, Carnedd Dafydd, Carnedd Llewelyn, Moel Siabod, Moel Hebog, 
and Snowdon. Gazing at this sea of mountains, the uninstructed 
geologist would find it difficult to believe that the rugged Snowdonian 
range that runs from Moel Hebog by Carnedd Llewelyn to Conway 
is composed of rocks which are the equivalents of the strata occupying 
the comparatively low-lying hills of the Bala district, east of Arenig ; 
but such is, nevertheless, the ease. 
It is impossible to give a perfect idea of all this unless in the country 
itself, or, at all events, with a geological map and sections before us ; 
but, in default of these aids, some idea of it may be obtained by the 
help of the accompanying panoramic diagram, drawn by Percy W. 
Hewgill, Esq., C.E., which my intimate knowledge of the country has 
enabled me to colour geologically. 
The view is taken from the summit of Cader Idris, which consists of 
an amygdaloidal greenstone, that slopes down the mountain towards 
Llyn-y-Cae. Under it are masses of felspathic trap, and long lines of 
greenstone, interbedded with altered slates, forming the steep northern 
cliff of the mountain, and overlooking the high valley that lies between 
the cliff and Mynydd Gader. That valley itself is formed almost 
entirely of felspathic ashes and conglomerates, with interbedded lines 
of greenstone, the whole dipping under the igneous rocks of the cliff. 
The same rocks stretch north-east, and form the peaks of Aran 
Mowddwy, and Aran Benllyn. Underneath these lie the Lingula-flags 
— so called from the prevalence in them of that oldest known brachiopod, 
