THE TRAPPEAN ROCKS OF K1LLARNEY. 
173 
tloned by Mr. Foot, is exactly like that to be seen in similar fel- 
stone in the neighbourhood ofPenmachno (North Wales). The 
nodules are compact, not different from the mass of the rock, al¬ 
though easily separable from it. 
2. A granular crystalline variety, in which the mass is composed 
of small crystals of felspar, and here and there of little crystalline 
particles of quartz. The crystalline particles of quartz frequently 
assume a perfectly rounded and amygdaloidal form, as if they had 
been crystallized in small pores or vesicles that had been produced 
in the mass; but they likewise occur in irregular crystalline forms, 
entangled among the crystals of felspar. 
In these two varieties the overplus of silica originally contained 
in the fused mass, beyond that necessary to form trisilicates with 
the alumina, potash, or soda, or whatever other bases the rock may 
contain, has either remained mechanically mixed in the paste of the 
general mass, or has crystallized out, as it could on the formation of 
the felspar crystals. 
3. The third variety is a bluish-gray felstone porphyry, with 
numerous crystals of white or pale flesh-coloured felspar, often as 
much as one-eighth or even one-fourth of an inch across. Crystal¬ 
line portions of quartz may occasionally be seen also in this. 
In some parts of this rock the crystals of felspar, although ex¬ 
hibiting its characteristic cleavage and lustre, have yet a rounded 
outline, either oval or almost circular. These must either have been 
formed in vesicular cavities, like the quartz mentioned above, or else 
the rock must be an altered “ ash,” and the felspar crystals have 
suffered from mechanical attrition before they were deposited in the 
mass. The ashy appearance of this rock is in some cases increased 
by its flaky character, but this is in many places perceptible where 
the flakes do not split parallel to the bedding of the adjacent slates, 
but to their cleavage , even when that is very different from the bed¬ 
ding. It looked as if the trap rock were affected by a concealed 
cleavage, which partial decomposition or weathering brought out , and 
gave the rock its flaky character. 
4. A fourth variety of the trap, generally found in its upper 
portion only, was an undoubted “ash,” a perfectly fine-grained, 
soft, flaky rock, smooth, with rather a soapy feel; easily ground to 
powder; of a pale greenish tint; translucent at the edges, its pow¬ 
der being nearly white. 
