3840 Prof. Grenville Cole—The Rocks of Rhobell-Fawr. 
phyritic pyroxenes remaining sufficiently preserved for recognition ; 
but brown hornblendes, lem. or so in length, have been brought 
up freely with it and form its more obvious porphyritic con- 
stituent. As usual, the hot magma has acted on the outer zone 
of the crystals, and has left only a band of magnetite dots; inside 
this is a zone in which the colour and pleochroism of the mineral 
are preserved; and the central area is bleached and fibrous, still 
retaining enough continuity to show that its extinction agrees with 
that of the brown zone. MHpidote has arisen in this area, and fine 
needles of almost colourless secondary amphibole penetrate the 
groundmass of the rock in every direction. The specific gravity of 
this hornblende-augite-andesite is as high as 2:95. 
III. Tue Tracuytic ANDESITES. 
Rhobell Fawr is a complex area of ridges and hollows, stream- 
cuts, and bolder valleys; and it was not until my third visit that 
I realized the important part played in the constitution of the 
volcano by rocks first noted on the extreme east, but having their 
full development on the passes over to Llanfachreth on the west.. 
These form a series of compact grey rocks resembling phonolites 
or tephrites in various stages of decay. Coming as I did on this 
last occasion fresh from the fascinating phonolite domes of N.W. 
Bohemia, the appearance of these rocks was most attractive, and 
their detailed study, as usual, disappointing. What felspathoid 
may have once been present in them, besides the abundant plagio- 
clase, it is now impossible to say. The felspars are largely converted 
into microcrystalline dusty areas; the porphyritic pyroxenes, pre- 
serving their outlines, are often mere patches of calcite. These 
plagioclastic lavas are unlike the Welsh “ felstones”’ on the one hand, 
and the heavy basaltic andesites on the other. They have now a 
pretty uniform specific gravity of 2°75, the more highly silicated 
eurite of Dduallt, which lies in the Arenig series, giving only 2°60. 
I have classed these grey rocks of Rhobell together under the 
Trachytic Andesites, since their common characters, poverty in 
ferromagnesian constituents and abundance of plagioclase, would 
certainly carry them into this division. 
Wherever I have been able to follow them out, these lavas are 
intrusive. One of them comes up between ash and slates under 
Graig Fach, and has baked the slates beneath it; its specific gravity 
is 2°74. Another, scoriaceous but intrusive, its hollows filled with 
chlorite, appears similarly above the slates half a mile to the north 
north-east of Graig Fach and south-west of Ty-newydd-y-mynydd. 
It has a specific gravity of 2:77. 
South-west of Graig Fach, and still at the base of the mountain, 
we have an intrusion of grey andesite among the slates, which are 
there well exposed along a steep stream-cut. This mass climbs for 
some distance towards the greenish ashes of the upper slope, and 
probably penetrates them, like the rock of Graig Fach. It contains 
some pretty pseudomorphs after zoned porphyritic pyroxene. 
In the hollow between Moel-Cors-y-garnedd and Rhobell Fawr 
