E. B. Tawney—Woodwardian Laboratory Notes. 17 
TV.—WoopwarpiaAn LAaBporatory Norrs—N. Wates Rocks IV. 
By E. B. Tawnry, M.A., F.G.S. 
(Continued from Dec. Il. Vol. TX. 1882, p. 553.) 
[Cl. 41]. Sedgwick Collection, from near the top of Bwleh Mawr, 
is a rock of greenish-grey colour, with rather fewer felspars than the 
last. In the microscope the ground is seen to be largely occupied 
by squarish crystals of felspar, between which is some crypto-crystal- 
line matter. The larger felspars seem all plagioclase, but they are 
all much decomposed, some of the smaller ones are perhaps ortho- 
clase. ‘There is no pyroxenic mineral preserved; calcite aggrega- 
tions result, and also viridite extinguishing in no position. There are 
many minute quartz grains in the ground, which have the appearance 
of being secondary. Apatite is abundant; there are also black and 
brown crystals of iron oxides (ilmenite). A few crystals of epidote 
have been formed. No hornblende is to be found, and amorphous 
matter in the ground is uncertain, yet the rock may most conveniently 
be placed among the porphyrites. It has not, however, the andesitic 
structure of the Carn Boduan rock. The dark colour would be due 
to the chloritic matter and iron oxide scattered through the ground. 
The hills Bwlch Mawr, Girn Goch, Girn Ddu, and Moel Penllech, 
form one continuous mass as mapped by the Geological Survey, their 
line is about parallel to that of the coast. ‘They all belong to one 
series of igneous action, being lithologically allied. Some of the 
stone in the “set” quarries on Girn Ddu is almost exactly like that 
of Bwlch Mawr, while in the same quarry it assumes sometimes 
quite a different colour and appearance, instead of dark grey, being 
pale pinkish or greenish. There is no trace of conglomerate or of 
inclosed fragments of them in the Cambrian shales which surround 
them. Their appearance, therefore, is later, their intrusive nature 
being inferred from their relations to the shales which surround 
them. ; 
Girn Goch, summit.—The rock here is of pinkish-grey colour, much 
weathered, as is natural on the top of an exposed peak. Under the 
microscope the ground is granulo-crystalline, consisting of quartz 
and felspar; the quartz abundant, among its inclusions are fluid- 
cavities with moving bubbles. Scattered through the ground are 
aggregates of viridite which under crossed Nicols depolarizes feebly ; 
with it are mixed up numerous epidote particles; these may be 
inferred to be the remains of hornblende which has entirely dis- 
appeared. The felspars of the rock, from the value of the extinction 
angles, and their streaky nature, are indicated as chiefly microcline. 
The rock may be classed as granite-porphyry. 
Girn Ddu, summit [Cl. 47], Sedgwick Collection.—Very like the 
prevailing rock of Bwlch Mawr, a dark grey ground, from which 
stand out numerous greenish-white felspars. Segregations of 
epidote visible to the unassisted eye; the triclinic nature of many 
felspars is detected with a hand lens. Under the microscope the 
DECADE II.—vVOL. X.—NO. I. 2 
