32 MR. F. RUTLEY ON AN ALTERED [Feb. 1902, 
read before this Society, I had occasion to allude to a brown 
substance occurring in a ‘quartz-blow’ from Waihi. This substance 
was a source of considerable perplexity to me; some of it formed 
spongy-looking fragments, which I doubtfully suggested might be 
fragments of vesicular lava and, inadvertently, it was positively so 
named in the description of one of the figures. In other cases the 
brown matter formed irregular streaks like whip-lashes. On my 
subsequently submitting a section of the rock to Dr. G. J. Hinde, 
F.R.S., he kindly examined it, and informed me that the brown bodies 
were doubtless of organic origin, but that he could offer no further 
opinion about them on account of their obscurity as regarded 
structure. The specimen alluded to was numbered H,,, and, in 
view of Dr. Hinde’s opinion, I beg publicly to recant the statement 
that fig. 4, pl. xxxii,' represented ‘fragments of brown glassy 
vesicular lava,’ and to say that it represented ‘ brown matter of 
organic origin in chalcedony ’ (* quartz-blow’) x 140. 
It Dr. Hinde’s opinion be correct, which I do not venture to 
doubt, the origin of the particular ‘ quartz-blow’ in question may 
have been somewhat similar to that of a geyserite or siliceous 
sinter; but I refrain from any further speculation about this rock, 
trusting that Prof. Park will work out further details concerning 
its mode of occurrence. 
After this digression, it may be well to say a few words con- 
cerning another rock from the vicinity of Builth. It is essentially 
a breccia, consisting of sharply angular pinkish to greyish-white 
fragments, embedded in a dark grey to black cement. Both the 
fragments and the cement are harder than steel, the point of a pen- 
knife making no impression. ‘This specimen, which so closely 
resembles a pumice-tuff or breccia from the sinter of Rotorua (H,,), 
was also given to me by the late Mr. H. W. Bristow, and was 
labelled as coming from Carneddau, near Builth. It and the speci- 
men first described in this paper were doubtless collected at the 
same time and, I should think, probably at or near the same spot. 
Provisionally I merely called it a felsite-breccia, and its general 
appearance in thin section under the microscope warrants such a 
name; but, when examined more carefully in ordinary light, most of 
the fragments show faint and occasionally fairly distinct traces of 
the fibrous structure and concave outlines so characteristic of the 
pumice-fragments, just described as occurring both in the Builth 
and Rotorua sinters. The outlines of such fragments are, however, 
obliterated in polarized hight, owing to their altered nature, in which 
alteration, judging from the extreme hardness of the fragments, 
impregnation and partial or entire replacement by silica must have 
played a very important part. Between crossed nicols the frag- 
ments show differences in texture, some being much more finely 
crystalline than others; the coarser seeming to have more the 
character of pumice-tuffs, while those of finer texture rather 
1 Quart. Journ. Geol. Soc. vol, lv (1899) pp. 463, 468. 
