202 
NOTE ON A ROCK FROM GLYN CEIRIOG. 
the China-ash bed, Pont-v-Meibion, Glyn Ceiriog, with a request 
that I would examine it and report the result. A variety of 
engagements has prevented a very detailed study, but two sections 
which I have prepared have furnished some interesting particulars 
as to the nature of the specimen. The mass was some five or six 
inches long and three inches thick. About two-thirds of this 
thickness was quite compact and solid and presented no special 
feature, but the rest was, to a large degree, made up of lenticular 
masses, varying in size from Jin. to Jin. in diameter, and Jin. to 
Jin. thick. The edges of the lenticles are, in some cases, sharp; 
in others, rounded off. These bodies are set in a compact matrix 
similar in appearance to the rest of the specimen, but more 
homogeneous. The way in which the rock fractures shows that 
the surfaces between the lenticles and the matrix are relatively less 
hard and compact than the mass of the rock. This makes it very 
difficult to prepare a satisfactory thin section, as the specimen has 
an unfortunate tendency to break up along the junctions, either 
during the grinding or on any attempt being made to move the 
slice to a fresh glass. 
The compact part of the specimen shows, under the microscope, 
that it is a volcanic ash or agglomerate of moderate grain, the 
fragments being composed of crystals of felspar and quartz, 
pieces of apparently a grey andesitic rock, some very perfect 
specimens of granophyre and flakes of slate, which lie with their 
shortest dimension perpendicular to the surface of junction with the 
other portion of the specimen, parallel, therefore, to the thickness 
of the lenticles. There is a good deal of what appears to be calcite 
scattered through the slide in irregular patches, which probably 
show the place of some other mineral in the original rock, though 
the boundaries are not sharp. This may partly be owing to the 
pressures to which the rock has no doubt been subjected, as evidenced 
by the dirty-looking lines in the section, which show the crushing 
and internal movements of the rock. There has been, however, no 
extensive crushing all through the substance of it; the twinning of 
the felspars is quite sharp and normal, strain shadows in the 
quartz grains are infrequent, and the structure of the granophyre 
fragments shows no trace of disturbance, 
September, 1893. 
