E. B. Tawney— Woodwardian Laboratory Notes. 67 
Comparing a slice of the Amwlch rock, we notice the same powdery- 
looking state of the quartz; this arises from the great multitude of 
inclusions— liquid inclusions with bubbles, many spontaneously 
moving; these are often in lines which stretch sometimes continu- 
ously across from one crystal to another. The quartz has formed 
evidently in presence of abundant vapours; its outlines are not 
always marked out by powdery matter; there seems to have been a 
thorough recrystallization. The felspars, both orthoclase and tri- 
clinic, are much decomposed. The dark substance in the intervals 
between the other minerals is not resolvable into any definite mineral, 
it seems a mixture of opacite and grey powder, sometimes coloured 
greenish from a trace of chloritoid matter; a little secondary epidote 
has been formed. 
Microscopic examination therefore shows that this is the same 
type of rock described by Prof. Bonney from Rhos Hirwain, Twt 
Hill, ete. It varies, however, in different parts of the area; thus the 
rock which one sees in Penygopan quarry, about 14 miles south- 
west of Sarn, appears more like a grey granite, since a dark horn- 
blendic material is abundantly present. 
A microscopic examination shows the quartz rather full of in- 
clusions, sometimes in lines roughly parallel, or even with two 
systems of lines transverse to each other. Among these are some 
with spontaneously moving bubbles, but bubbles seem not very 
abundant in these inclusions; the outlines of the quartzes are margined 
sometimes by kaolinized matter, but not always. The felspars are 
about as abundant as the quartz; most of them show multiple twin- 
ning, many are so decomposed that its absence may be due to con- 
version of the felspar into secondary aggregates, but probably some 
orthoclase may be inferred. The hornblende is also in a state of 
transition, a little of the original cleavage is left, but most of it has 
lost its dichroic property, and is a pale greenish substance, partly 
actinolite from its optic properties, and partly chlorite or viridite in 
masses much grouped together. A few scales of brown mica are 
seen, and brown iron oxide, probably formed at expense of the 
hornblende. Apatite is present. We may class it as hornblendic 
gneiss (if igneous, it would best be called quartz-diorite rather than 
syenite). The constitution of the rock, however, is against its being 
igneous, and its great variability within short distances also. Owing 
to the absence of openings, I was unable to find a contact with ad- 
jacent beds, except in the case above described. 
At Penllech, near by, the rock looks something like a fine-grained 
granite with minute ill-characterized mica. 
By the aid of the microscope it is seen that the quartzes are rather’ 
full of inclusions, among which a few spontaneously moving bubbles 
may be observed. The outlines of the quartz elements are often 
not to be distinguished except by the aid of polarized light. Of the 
felspars a large proportion is plagioclase; they contain similar in- 
clusions, but I could not detect any bubbles. Some biotite is 
present, but it gradually loses its strong absorptive and dichroic 
characters, becomes green, finally loses its cleavage, and so passes 
