NOTE ON A ROCK FROM GLYN CEIRIOG. 
203 
Unfortunately, the junction between the two parts of the 
specimen is a place of very weak cohesion, so that I could not 
prepare a slide comprising both parts, but the transition appears to 
be quite sharp. 
The first thing which strikes one in examining the other 
portion of the specimen is the fact that the greater part 
of the lenticles presents exactly the same characters as the 
matrix does, and that they differ markedly from the compact ash 
just described. The texture of this part is very similar to that of 
some of those felsites which are believed to have been originally 
glassy rocks, showing the irregular mingling of two slightly different 
magmas. Rutley describes such from Beddgelert and from the 
Pass of Llanberis, and compares the appearances presented by 
these with those of some still glassy Hungarian rocks. The 
beautiful rock of Schedewitz is also a case in point. 
Scattered about in this ground mass, however, there are crystals 
of both felspar and quartz, which are plainly fragmentary ; and, on 
the whole, I am rather inclined to suggest for the origin of the rock 
a fine volcanic dust mingled with broken minerals. The pressures 
to which the rock has been subjected were not at right angles to 
the bedding, but have tended to crumple the little layers into 
more or less irregular folds, filling up with secondary quartz and 
possibly felspar. The clearer parts break up when polarized light 
is used into a mosaic of fine grain, in which it is impossible to 
distinguish the actual nature of the mineral, or whether it is a 
mixture of quartz and felspar, as would appear not unlikely if it is 
really due to a process of devitrification in a glass. 
As previously stated, the greater part of the lenticular nodules 
show the same structure, but at the edges, i.e., at the sharper ends of 
the elliptical sections, there is evidence of pressure and of disintegra¬ 
tion by reason of it. The structure mentioned is quite lost, and is 
replaced by a very fine grained mass exactly resembling a typical 
mylonite, the fine powder resulting from the crushing having, as it 
were, flowed outwards to the edges of the nodules. These are also 
coated with a brilliant dark crust, very thin, giving the impression 
of a very slight coating of a mica or allied mineral. 
September, 1898. 
