Vol. 58.] SILICEOUS SINTER FROM BUILTH. 3d 
resemble fragments of altered siliceous sinter, with little, if any, 
tufaceous admixture. An occasional fragment indicates that other 
rocks, suggestive of highly altered porphyrites, have contributed 
fragments to this breccia. 
The cement which binds the fragments has the appearance of 
vitreous matter rendered turbid by brown stains and dark streaks 
and spots, the latter appearing in quite irregular forms when 
examined under an amplification of 250 diameters, and showing no 
approach to definite crystal-form. Under crossed nicols this cement 
appears more or less dark, but is filled with very minute birefringent 
specks. If the cement be regarded as possibly a siliceous sinter, 
and its hardness warrants such a supposition, it certainly has not 
attained the advanced stage of alteration of the sinter from Builth, 
previously described ; and since it cements fragments of rock, most 
of which resemble the sinter already alluded to, it must, therefore, 
be of later age. Both fragments and cement are also traversed by 
delicate veins of quartz, the latter marking a still later stage in the 
history of this rock. 
The specimen which it most resembles is a pumice-tuff or breccia 
(H,,) from Rotorua. The angular or subangular fragments of 
pumice are perfectly fresh, and the cement very slightly, if at all, 
exceeds in quantity that present in the breccia from Carneddau. 
Between crossed nicols a section of the breccia from Rotorua is 
practically isotropic, except for the hyalite which represents the 
greater part of the cement. Here the cement has a marked bire- 
fringence, and the small globules of hyalite show the usual charac- 
teristic dark crosses. ‘There are, however, some limited areas where 
the siliceous sinter has not assumed the condition of hyalite, and 
the sinter then appears isotropic, save for some minute birefringent 
specks which indicate the first stage of alteration. This section 
also contains some brown matter, probably of organic origin, and 
similar to that occurring in the other sections described. That this 
pumice-breccia from Rotorua would, in time, pass into the condition 
of that from Carneddau seems by no means unlikely. 
It is quite possible that further field-work may show that, in 
the Builth neighbourhood, conditions have existed similar to those 
which now exist in and around Lake Rotorua; yet, so far as I 
know, the recent sinters of New Zealand and the old sinters of 
Wales have not hitherto been compared. 
Tn conclusion, I wish to tender my sincere thanks to Mr. Frederick 
Chapman, A.L.S., F.R.M.S., for the lantern-slides and photographs 
which he prepared in illustration of this paper. 
PostscriPT. 
[When writing this paper I was not aware that my former 
colleague, Mr. J. G. Goodchild, F.G.S., had found sinters, both 
siliceous and calcareous, in the volcanic rocks associated with the 
Lower Carboniferous System in and around Arthur’s Seat. To 
Q. J.G. 8. No. 229. | D 
