28 MR. F. RUTLEY ON AN ALTERED [Feb. 1902, 
2. On an AxrerED Stiiceous SintER from BuittH (BRECKNOCK- 
SHIRE). By Franx Rurvey, Esq., F.G.8. (Read November 6th, 
1901.) 
[Puare IL] 
In a former paper some tufaceous rhyolites from Dufton Pike were 
described,’ which seemed to afford evidence of solfataric action, yet 
they scarcely gave the convincing proof that could have been desired 
of the former existence of hot springs and geyser-action among the 
old volcanic districts of the British Isles. 
Attracted as I was by the resemblance of a small specimen of a 
brecciated rock from Carneddau, near Builth, to a rock from Rotorua, 
described in a former paper,” it seemed desirable to examine other 
small chips from the Builth neighbourhood which were given to me 
many years ago by the late Mr. H. W. Bristow, F.R.S., my former 
Director on the Geological Survey. Among these specimens was one 
labelled ‘ Rock acted on by heat, Builth,’ which seemed to promise 
more satisfactory information on the subject of old siliceous sinters 
than that to be derived from the microscopic examination of the 
generality of quartzites, the latter often showing incontrovertibly 
that they are sandstones which have undergone various degrees of 
alteration. 
The rock now to be described shows no trace of original sand- 
grains. It is unfortunate that the label gives no indication of the 
precise locality at or near Builth from which the specimen was 
derived.* Apart from this drawback, however, 1t appears to be of 
sufficient interest to deserve record. If the deposit be of tolerable 
thickness and extent, its pale yellowish-white or pinkish-cream 
colour should render it easy of recognition by future investigators 
of the geology of the district. 
- + rock has an almost compact or an exceedingly fine-grained 
character, and a fracture which is in some parts coarsely platy, in 
others small-conchoidal. It is veined with quartz, sometimes dis- 
tinctly crystallized where quartz was apparently attached to the 
walls of slightly gaping fissures. Both the rock and the veins 
which traverse it are too hard to be scratched by a knife-point. 
Small splinters of the rock can, however, be fused on thin edges 
to a white, frothy glass, after long exposure to the blowpipe- 
flame. 
Small splinters from a white siliceous sinter (H,,) from Rotorua * 
proved under like circumstances to be quite infusible, but in this 
specimen the silica was found by Mr. Philip Holland to amount to 
93-59 per cent. In siliceous sinters, also from Rotorua (H,, and 
1 Quart. Journ. Geol. Soe. vol. lvii (1901) p. 31. 
2 « Additional Notes on some Eruptive Rocks from New Zealand’ Quart. 
Journ. Geol. Soe. vol. lvi (1900) p. 493. 
? It probably came from Oarneddau or its vicinity. 
* Op. cit. p. 503. 
