Vol. 58. | ALTERED SILICEOUS SINTER FROM BUILTH. 31 
fragments in the rock from Builth break up into a mosaic of bi- 
refringent grains under like examination. ‘This is shown in Pl. IT, 
figs. 7 & 8, the upper figure depicting a fragment of pumice in the 
Builth rock, aS seen in ordinary srenismibted: light: while the lower 
_ figure represents the appearance of the same spot in the section, 
when viewed between crossed nicols. No trace of the fragment of 
pumice is any longer to be seen in the latter figure, but merely a — 
confused mosaic of birefringent grains, the pumice being altered 
into the same condition as the Sinead ite siliceous sinter. 
In Pl. I, fig. 1, is represented the general appearance of the Builth 
rock in ordinary transmitted light; while fig. 2 shows the same part 
of the section when viewed between crossed nicols. In the same 
Plate, part of a section of pumice- -charged geyserite or siliceous 
sinter from Rotorua is shown for comparison, fig. 3 representing 
what is seen in ordinary transmitted light, while fig. 4 shows the 
same part of the section as it appears between crossed nicols. 
The fragments of pumice in fig. +, being in an unaltered conde 
are quite “dark, while no such “fragments are to be seen in fig. 2 
since the fragments of pumice in the Builth rock are as much altered 
as the sinter surrounding them. It should also be noted that the 
sinter in the New Zealand rock appears to be neither so dark nor 
so highly crystalline (between crossed nicols) as the sinter in the 
Welsh rock. In these figures we compare, in fact, the appearance 
of a young sinter with that of an extremely old one. 
There yet remains another feature in these rocks about which it 
might be well to say as little as possible. Some of the siliceous sinters 
of New Zealand contain small patches of a brown substance, 
which in the thicker parts of a section appear very dark brown or 
nearly black; and in the rock from Builth, which we have been 
trying to describe, there are patches of what seems to be a similar 
substance. Mr. Frederick Chapman has kindly examined some of the 
sections of this rock from Builth, but, although he believes that the 
patches represent something of an organic origin, he has failed to 
detect anything which will enable him to say whether it belonged 
to the animal or to the vegetable kingdom. 
Since Prot. Weed noted the presence of algous growths in some 
of the siliceous sinters of New Zealand," it seems possible that this 
brown substance may also be of a like nature. <A patch of the 
questionable brown substance is represented in the middle of fig. 5, 
Pl. II, which has been drawn from a section of the Builth rock; 
while fig. 6 in the same Plate shows part of a section of pumice-tuff 
cemented by siliceous sinter from Rotorua, in which several patches 
of this brown substance are represented. In this case they are 
clearly seen to fill up spaces between the fragments of pumice 
The brown substance in the Builth rock seems to occur in an equally 
irregular manner. 
It is only right that I should mention that, in a previous paper,” 
1 U.S. Geol. Surv. 9th Ann. Report (1887-88). 
2 “Notes on the Rhyolites of the Hauraki Goldfields’ Quart, Journ. Geol. 
Soc. vol. lv (1899) p. 449. 
