366 Proceedings of the Royal Society 
on a plain consisting of gneiss and mica-slate; that the base of the 
mountain itself is a beautiful granite, or rather syenite; that this is 
succeeded by a bed of pale felspathic porphyry, consisting of a pale 
reddish matrix of compact or amorphous felspar, with numerous 
crystals of still paler felspar; that, when we arrive at the base of 
the great North-Eastern Precipice, this rock acquires a greenish- 
black colour, apparently owing to the felspathic matrix becoming 
“ impregnated with the matter of hornblende ”; that the whole 
immense mass of the precipice, a mile and a half in extent, and 
1500 feet in height, is composed of this dark porphyry, variously 
modified; and that sometimes the porphyrising crystals of pale 
felspar are so scanty, if not altogether wanting, that the rock 
becomes difficult to distinguish from basalt or clinkstone. 
This description may be applied to the structure of the mountain 
in the present day ; but a more recent geology looks differently at 
the composition and nomenclature of its rocks. I have not seen a 
later publication on this subject. But Mr. Clement L. Wragge has 
favoured me with a perusal of a MS. account of it by Professor 
Judd of London, who writes from recent observation, and whose 
views agree with those of other geologists whom I have consulted. 
The whole mass of the mountain is a volcanic protrusion through 
the gneiss and mica-slate of the plain below on which the mountain 
rests. I may here observe in passing, that, as this substratum has 
a dip from south to north, mica-slate shows itself as part of the 
mountain at its southern face, where it forms the steep rugged north 
slope of the second reach of Glen Nevis. Here there is an extensive 
lofty terraced cliff, consisting of a tough fine-grained mica-slate, — 
if one may judge from examination of the enormous blocks which 
strew the bottom of the cliff, or the nature of the fixed beds that 
underlie it. The base of the mountain itself is granitic porphyrite; 
the porphyrising ingredient being hornblende in numerous crystals. 
The whole prodigious mass above, composing the great North Eastern 
Precipice, is felstone porphyrite, coloured greenish-black by the 
admixture of some other mineral, sometimes assuming the external 
characters of a simple porphyry by porphyrisation with crystals of 
greyish-red felspar, — often putting on the characters of volcanic tuff 
or breccia owing to disappearance of the felspar crystals more or 
less, — and then occasionally becoming so dark that its structure as 
