Before treating of the subject before me, I must describe its history. At a soirée of the Royal Society on April 25, 1877, I showed a number of specimens illustrating a new class of optical properties, applicable to the identification of minerals. These greatly interested the late Sir G. G. Stokes, Bart., and led to a considerable amount of correspondence between us ; and he communicated a paper to the Royal Society on the mathematical part of the subject, and I sent a short one on my apparatus and observations. This was admitted to be very imperfect, since the crystals examined, though showing the general facts perfectly well, had not been cut so as to be suitable for such quantitative measurements as could be compared with Stokes's theoretical determinations.

This occurrence of barytes is somewhat remarkable and interesting from several aspects :—The peculiar conditions under which it seems to have been deposited, the limited quantity, and the perfection and richness of the crystallographic development.

It was found some two years ago in one of the shale pits of the Young's Paraffin oil Company, and my friend Mr. J. S. Thomson, to whom I am indebted for two of the four or five existing specimens (of which one, I am happy to say, has been deposited in the Museum of Edinburgh), was so kind as to procure for me from the Mining Engineer of the Company a sketch of the Geological Strata of the neighbourhood, in which the position of the beds containing the crystals is indicated (Plate II.).

In a description of some British pseudomorphs, published in 1896 (Mineralogical Magazine, XI, p. 272), I mentioned that the sky-blue and lavender-blue mineral sometimes found encrusting the pyromorphite from Roughten Gill is not a zinc carbonate or silicate, as commonly supposed, but is a phosphate of lead and aluminium. Pseudomorphs of this substance after pyromorphite are not rare among Roughten Gill specimens, and these were described in the above mentioned paper as examples of plumboresinite after pyromorphite.

The perthite megacrysts from charnockitic and non-charnockitic members of a sialic intrusive complex in the Varberg area of south-west Sweden have been investigated by powder X-ray diffraction. The extent of Al/Si order in the K-feldspar host lattices of the megacrysts is restricted in all but the major alkali granite differentiate. A direct relationship between the hydration state of the containing rock and the degree of order of the megacryst lattices is indicated. High lattice strain is common to all the megacrysts but decreases progressively with increasing triclinicity.

The chemistry and petrography of a meta-ultrabasic lens containing peridotite with poikilitic pyroxenes and hornblende, both of probable igneous origin, are given. The body also contains feldspathic-bearing hornblende meta-gabbros and is a tectonic fragment in hornblende-plagioclase rock belonging to the same Errismore intrusion. Chemical analyses fail to discern any appreciable systematic differentiation trend across the body despite modal variation to hornblende plagioclase layers. The high Niggli mg near the structural bottom of the intrusion suggests that this part of the Errismore intrusion is right way up, contrary to a previous suggestion.

Sedimentary analcime in a small basin in Cainozoic basalt at Murrurundi, New South Wales, has probably resulted from the reaction of Na-rich water on colloidal nontronite derived from volcanic ash. Based on cell dimension (13.713 Å) and Δ2θ(1·81) determined by Saha's (1959) method it is a Group C low-silica type of Coombs and Whetten (1967).

This investigation investigates geologically old, ca. 370 Ma, metamict zirconolite from the Kovdor phoscorites and carbonatites in the Kola Alkaline Province. Mineral composition, crystallisation behaviour, and thermal expansion of the recrystallised samples were analysed using electron microprobe analysis, Raman spectroscopy, and in situ high-temperature powder X-ray diffraction (HTPXRD). The zirconolite crystals investigated are different in their morphology, internal texture, composition, alteration degree, and can be divided into four distinct groups. The zirconolite is a high Nb and Fe3+ variety (10.8–24.1 wt.% Nb2O5 and 7.9–9.0 wt.% Fe2O3), it is enriched in Th (up to 8.7 wt.% ThO2), Ta (up to 5.3 wt.% Ta2O5) and rare earth elements (up to 5.0 wt.% REE2O3). Raman spectroscopy confirmed that metamict zirconolite is anhydrous.

The recrystallisation process of the metamict zirconolite is complex, as detected by HTPXRD. A fluorite-type phase starts to crystallise at 420°C. The formation of a pyrochlore phase can be identified at 750°C. The major phases detected in the sample after the recrystallisation are: zirconolite-3T (53 wt.%), srilankite (25 wt.%), pyrochlore (15 wt.%), baddeleyite (5 wt.%) and zircon (3 wt.%). The average coefficients of thermal expansion (CTE) values in the temperature range 25–1200°C are as follows: ${{\bar \alpha }}$a = ${{\bar \alpha }}$b = ${{\bar \alpha }}$11 = ${{\bar \alpha }}$22 = 8.95·10–6 deg–1. Similarly, the thermal expansion along the c-axis yields a similar value: ${{\bar \alpha }}$a = ${{\bar \alpha }}$b = 8.93·10–6 deg–1, indicating an almost isotropic thermal expansion of zirconolite-3T. The lower CTE value compared to a pure synthetic zirconolite observed for zirconolite-3T might be attributed to the complex chemistry and polyphase nature of the material investigated.

Cliftonite was described by Sir Lazarus Fletcher in 1887 as ‘a cubic form of graphitic carbon’. He put forward reasons for regarding it as a new allotropic modification of carbon and other reasons suggesting that it is a pseudomorph of graphite after some cubic mineral, perhaps diamond, but did not definitely favour either view. On the evidence available hitherto, both suggestions were clearly possible.

In a paper read before this Society on June 16th, 1885, Mr. Wallace drew attention to this mineral as being found at Loch Bhruithaich, and stated that it was obtained incrusting Barytes.

The mineral is clear, beautifully crystalline, with a tendency to a bluish tinge of colour, and very friable.

Spectral reflectances and chemical compositions of twenty-eight different grains of exsolved titanomagnetite were measured. Relations between reflectance and composition were established by calculating best-fit planes (Kummell, 1879) for the reflectance at 499 and at 617 nm. These were then transformed to expressions from which the contents of Ti and Mg can be estimated. Even though these estimates are not as accurate as microprobe values, the method is still a useful tool in the study of chemical variation in exsolved titanomagnetites.

A nordmarkite from the Ben Loyal syenite complex has been analysed, in which a powdery yellow mineral occurs in miarolitic cavities. The mineral may be a new hydrated species with a monazite structure and a chemical composition essentially that of monazite but with silicon partly replacing phosphorus and with magnesium, calcium, and iron partly replacing the rare earths.