456 Notices of Memoirs—The British Association. 
examined optically by Prof. Rosenbusch, who writes as follows: ‘Tt 
was determined on more than 20 cleavage-flakes, that the axis of 
elasticity which makes an angle of only about 5° with c is a, not y, 
as in the [other] amphiboles; its colour is dark blue; b=, a rather 
less deep blue; y, which is almost perpendicular to c,—green. Axial 
angle large. The characters are thus surprisingly like those of 
aegirine among the pyroxenes.” That riebeckite holds chemically, 
as well as optically, the same place among the amphiboles as aegirine 
does among the pyroxenes, appears from Sauer’s analysis. Compared 
with arfvedsonite, it shows more silica, much more ferric, and less 
ferrous oxide. Taking account of all its characters, there can be no 
reasonable doubt that the Mynydd Mawr mineral is riebeckite. 
The blue tourmaline which I described, not without misgivings, 
as accompanying the blue hornblende, is, in all probability, the same 
mineral. Owing to the almost opaque nature of the sections, I was 
reduced to experimenting on the minute and impure crystals with a 
knife and a candle-flame ; so perhaps the mistake was a pardonable 
one ; the more so, as Professor Bonney, who has pointed it out to me, 
appears to have been himself deceived by the Socotra riebeckite, 
regarding it as a pseudomorph of tourmaline after hornblende.’ His 
figure closely resembles the slides from Mynydd Mawr. 
Besides the larger crystals of riebeckite, Sauer finds in the Socotra 
granite microlites of the same mineral, precisely similar to the 
colourless and pale-blue microlites already described in the rock oi 
Mynydd Mawr. That these belong to riebeckite, rather than to 
tourmaline, is proved by the direction of maximum absorption being 
parallel, not perpendicular, to the long axis. Sauer regards them as 
due, at least in part, to the secondary alteration of the felspar. Ths 
last suggestion cannot be maintained in the case of the Welsh rock. 
Apart from chemical difficulties, the mode of occurrence of the 
microlites, and especially their fluxional disposition through the 
ground-mass, show them to be original constituents of the rock. 
NOTICES OF MEMOTRS. | 
I.—British Association FoR THE ADVANCEMENT OF SCIENCE. 
Bata Merrine, Septemper 5TH To 121Tu, 1888. 
List of Titles of Papers read before Section C. Geology. 
Professor W. Boyp Dawkins, M.A., F.R.S., F.G.S., President. 
The President’s Address. (See infra p. 459.) | 
Horace B. Woodward.— Further Note on the Midford Sands. (p. 47(C) 
Horace B. Woodward.—The Relations of the Great Oolite to tie 
Forest Marble and Fuller’s Earth in the South-west of Hnglan. 
(See infra p. 467.) 
Horace B. Woodward.—Note on the Portland Sands of Swindon, ec. 
(See infra p. 469.) 
O. W. Jeffs.—On Geological Photography. 
1 Phil. Trans. vol, clxxiy. part i. p. 283, 1873. 
