ole T. G. BONNEY.ON THE BASE OF THE 
Cwm-y-Gilo village. Here the streaky structure is as distinct as in 
a modern rhyolite, and the platy jointing is hardly less so. In parts 
of the quarry it is a true “fissile structure” *, cutting the fluidal 
lines at various angles, sometimes as much as 70°. 
(2) On the western side of Llyn Padarn there is a band of slate 
intercalated in the felsite. This is a rather soft black slate, not 
conspicuously altered. We found exposures (probably of the same 
band) both by the roadside and in the railway-cutting. The former 
is only about 3 feet wide, and from its appearance might be claimed 
as an included fragment; but I have elsewhere seen soft rocks 
similarly “nipped” by interbedded harder strata. Microscopic 
examination shows the structure of the slate to resemble that of the 
eround-mass of some of the chiastolite slates: the junction with the 
felsite suggests that the one rock has been deposited on the other; 
for the material of the slate can be traced filling a crack m the 
felsite, the surface of which seems as if it had been denuded. 
(8) Near Llys Dinorwig, north-east of Llyn Padarn (lower end), 
the felsite is associated with an agglomerate. This was shown to 
me by Prof. Hughes, who at the time was not quite satisfied as to 
its true significance. On that point, however, there is no reason for 
doubt. It is as characteristic as any that are seen in Charnwood, 
containing fragments of all sizes, from mere lapilli to blocks full 
2 feet in diameter. One or two, indeed, may possibly be the ends of 
small “strings” of lava. Here and there a band of finer material is 
interstratified, as in recent volcanic cones. I have also examined 
this under the microscope, and find it made up of rhyolitic lapilli, 
some showing flow-structure very well, with fragments of quartz and 
felspar imbedded in an altered felspathic dust—in short, the usual 
structure of the matrix of such an agglomerate. The mass appears 
to be intercalated in the felsite near to its northern edge; but time 
did not allow me to trace out its boundary with any minuteness. 
Proceeding, then, to the examination of a series of specimens, we 
find that the rock in the neighbourhood of Llyn Padarn is a compact 
felsite of a dull grey colour, sometimes with a reddish tinge, por- 
phyritic with numerous quartz grains and small whitish crystals of 
felspar. As a rule, there is nothing abnormal in its appearance ; 
some fragments could hardly be distinguished from rhyolites from 
Hungary or the Euganean hills. I have had a series of six slides 
prepared to exhibit the principal varieties. 
(1) Quarry North of Cwm-y-Glo.—This rock, as already stated, 
shows flow-structure remarkably weil. Under the microscope it 
resembles a glass of slightly granular structure, with wavy light- 
brown bands, occasional clear rather granulated interspaces, and 
clouds of disseminated opacitet. With crossing Nicols the granular 
part is seen to consist of minute crystals. With a high power these 
are shown to be partly extremely minute colourless belonites, partly 
* See my paper, Quart. Journ. Geol. Soc. vol. xxxii. p. 142, A glance shows 
it is not a true cleavage, as it frequently changes its direction, 
+ For example, in thin bands of soft schists intercalated in the granitoid gneiss 
of the St, Gothard., t Plate XIII. fig. 1. 
