170 A. R. Andrew—The Doilgeliey Gold-belt. 
baked shales project from the softer unchanged shales, and also as a rule 
from the intrusive rocks themselves. The black shales are to a certain 
extent decolorized by the baking. 
I believe that in this district the 1 igneous rocks all belong practically 
to the same period. ‘There is no great diversity of lithological 
character among them, and they probably originated in the same 
common magma. ‘The petrographical character of the igneous rocks 
is difficult to determine, the metamorphic action to which the rocks 
have been subjected has been sufficient to destroy and change the 
characters of many of the minerals of which they are composed. 
There are two types of igneous rock represented in the district : 
(1) Diabase (of Harker, Teall, etce.); (2) Porphyry (of Rosenbusch). 
Among the diabases are practically all the ‘greenstones’ of 
H.M. Geological Survey. (The name ‘greenstone’ is so convenient 
for referring to the diabases that I have used it almost invariably 
throughout this paper.) The structure of the rock is intermediate 
between the plutonic and the volcanic types, though there is con- 
siderable variation and gradation in structure. The minerals that 
occur in the rock are oligoclase, hornblende, uralite, pyrrhotite, 
pyrites, chalcopyrite, calcite, sericite, kaolin, and iron oxides. Plagio- 
clase forms the most conspicuous phenocrysts in the rock; the 
erystals, rounded in outline, are twinned—chiefly albite twins; the 
plagioclase is probably a basic oligoclase close to andesine; the felspar 
phenocrysts are always cloudy, the cloudiness being due to the 
presence in them of weathering products such as calcite, kaolin, etc., 
the individuals of felspar range down in size to minute lath-shaped 
crystals, which make up a large proportion of the ground-mass of the 
rock. Hornblende is seldom seen to satisfaction, it occurs among 
the phenocrysts, where it is intensely brown and strongly pleochroic; 
usually the hornblende is completely decomposed into sericite and 
chlorite. Pyrites, pyrrhotite, and chalcopyrite are very abundant 
in the rock, but as far as my examination goes, there is no evidence 
to determine whether these minerals were originally present in the 
rock or have been subsequently introduced. Chlorite, sericite, 
kaolin, and iron oxides are found among the decomposition products 
of felspar and hornblende, but their characters call for no special 
mention. The specific gravity of this diabase rock is 2°84; it should 
be noted that the felspar is oligoclase, which is rather too acid a type 
of plagioclase to be usually found in a diabase. 
The second type of the igneous rocks of this Gold-belt is a porphyry 
or uralite porphyry; it is of much more restricted occurrence than 
the diabase, and I know of only two outcrops where fresh specimens 
may be obtained; these are at Y Garn and at Cefn-deuddwr. The 
Y Garn rock is typically porphyritic, the phenocrysts consisting of 
orthoclase, uralite, and hornblende, the latter much decomposed ; the 
ground-mass is fine-grained and microcrystalline, and is densely 
packed with little squares of orthoclase, which, like the phenocrysts, 
present only the usual characters. The only other minerals in the 
rock are muscovite, chlorite, and pyrites, produced by the decom- 
position of the hornblende; the specific gravity of the rock is 2°738. 
The Cefn-deuddwr rock differs from that occurring on Y Garn in 
