306 T. G. BONNEY ON THE MICROSCOPIC STRUCTURE OF 
felspar, its chief constituent, is much decomposed, and some of the 
quartz seems of secondary formation, it is difficult to pronounce. 
‘I'he general form of the felspar suggests that a plagioclase predomi- 
nates over orthoclase. A fair amount of opacite is present, and one 
or two scales which may be an iron mica. I haye no doubt the rock 
is ali igneous one, but hesitate whether to call it a quartz-felsite or 
quartz-porphyrite. 
V. Nanhoron quarries (p. 299). 
(a) Structure generally rather like III., except that a microscopic 
graphic, sometimes almost dendritic*, structure is common. Felspar 
decomposed, but some Carlsbad twins still recognizable, so probably 
mostly orthoclase. The dark finely granular mineral visible in the 
hand specimen is seen, under the microscope, to be chiefly a pale yel- 
lowish mineral, sometimes rhomboidal in outline, having a rather 
eranular structure and fairly distinct cleavage, associated with and 
often rather clouded by opacite. I believe it to be titanite, and do not 
find either mica or hornblende. The rock is igneous, and is thus 
a kind of aplite. 
(6) The hand specimen of this rock has a general resemblance to 
the last, except that it is more compact, and the dark specks are 
blacker and more sharply defined; under the microscope the struc- 
ture is seen to be microcrystalline, consisting of quartz and felspar 
with a fair amount of iron peroxide (hematite in part) in clustered 
granules. The ‘ dendritic” structure more abundant here than in 
the last specimen ; no titanite is certainly recognizable, but neither 
mica nor hornblende are present. One or two larger crystals of 
felspar are orthoclase. The rock is a quartz-felsite. 
VI. Near Ty-mawr, Rhos Hirwain (p. 300). 
This rock exhibits the peculiar structure characterizing the grani- 
toid rocks as distinguished from the true granitesy. It chiefly con- 
sists of quartz, felspar, and mica; the first is so full of minute enclo- 
sures as to look quite ‘grimy.’ The felspar also, besides being 
rather decomposed, often does not appear to be clear. Orthoclase 
is present, some cross-hatched, as described by Mr. Rutleyt; also 
some very closely twinned plagioclase. There is a fair quantity of 
* See description of this structure, Geol. Mag. dec. 2, vol. iy. p. 508. 
+ The distinction of structure between the granites and granitoid rocks 
(that is, between those of igneous origin and metamorphic clastic rocks) is a sub- 
ject to which I have paid much attention, and on which I hope in course of time 
to communicate some notes to the Society. In the latter, for example, the 
quartz differs from that in true granite, sometimes by showing a more suban- 
gular and fragment-like outline, sometimes by a much more irregular boundary. 
‘The grains also vary move in size and are more irregularly distributed ; the 
felspar also differs in form, exhibiting corresponding irregularity. In parts of 
the slide a little of an indefinite felsitic matrix may be seen. Other differences, 
which cannot be briefly described, exist ; but for the present it may be enough to 
say that my microscopic studies have strengthened my scepticism as to the ex- 
istence of “metamorphic granite.” 
{ Quart. Journ. Geol. Soc. vol. xxxi. p. 479. 
