Vol. 58.] SILICEOUS SINTER FROM BUILTH. 29 
‘H,,),! containing many small fragments of pumice and some of felspar, 
fusion on the edges of thin splinters could, on the other hand, be 
clearly effected in the flame of the blowpipe: a circumstance easily 
to be accounted for by the smaller amounts of silica =81-°99 and 
81:22 per cent., which these specimens were respectively found to 
contain. From its slight fusibility, it is clear that if this Builth 
rock be a siliceous sinter, it 1s a more or less impure or tufaceous - 
one, containing small fragments of fusible minerals or rocks like the 
specimens H,, and H,, from Rotorua. 
Two other possibilities presented themselves, either that the rock 
was a chert or that it was a lithoidal rhyolite. The former 
supposition is negatived by the absence of any fragments of shells, 
erinoids, sponge-spicules, or foraminifera, such as one usually meets 
with in cherts, by the absence of rhombohedral crystals of carbonates, 
and also by the absence of the radial crystallizations frequently to 
be seen in cherty deposits. Furthermore, that this rock is not allied 
to any form of rhyolite seems proved by the absence of any of those 
microscopic structures which so commonly characterize rhyolites and 
their altered representatives. 
Under the microscope, it is seen that the rock from Builth is 
decidedly tufaceous. It appears that this tufaceous matter is not 
entirely of one kind, since small pale greenish crystals, often with 
somewhat ragged boundaries, are here and there present in thin 
sections ; but they are generally so much altered and over- or under- 
lain by the small birefringent grains of the sinter, that it is difficult 
to ascertain anything definite about their optical properties. They 
often seem to give straight extinction, and may be epidote. 
By far the greater part of the tufaceous matter 1s, however, seen 
to consist of diminutive fragments of pumice. These, at first 
sight, are not very evident, being usually faintly outlined in pale 
brown, or they often appear as almost colourless phantoms of small 
pumice-shreds, their concave boundaries and their occasional 2 1ca- 
tions of fibrous structure alone serving to show their real character. 
In order to see them distinctly, it is needful to search the clearer 
parts of the sections in ordinary transmitted light, under a moderately 
high magnifying power, and to employ careful focussing. 
To ayoid any bias from a preconceived notion regarding the nature 
of these small bodies, a number of drawings were made from them 
and from fragments of pumice occurring in the siliceous sinter (H.,,) 
from Rotorua; and a comparison of.a selection of these rough out- 
lines reproduced in the accompanying figure (p. 30), will, I think, 
sufficiently prove that the resemblance between the fragments from 
the one locality to those trom the other justifies the assumption that. 
in general form at least, there is no essential difference. 
The resemblance ceases, however, when polarized light is employed. 
The fragments of unaltered pumice in the sinter from Rotorua 
are quite isotropic when viewed between crossed nicols, while the 
1 Quart. Journ. Geol. Soc. vol, lvi (1900) pp. 502 & 508. 
