Suppose that in the study of a mineral we have determined the cell-dimensions and the density, and made a ehemienl analysis. From these we can readily calculate the empirical unit-cell contents, which will usually approach integral values and so suggest a probable formula.

But the calculated empirical unit-cell contents, being based on chemical and physical data subjeet to experimental error, will be subjeet to numerous sources of error, and in any serious study it is very desirable to have some rough idea of the probable limits of error. Unfortunately, this is a matter of considerable complexity, but the following suggestions may be of assistance, though they will doubtless be considerably improved on.

Selfridge (1936) examined by chemical, optical, and X-ray powder methods a large number of specimens of serpentine minerals (ideal formula H4Mg3Si2O9), many of which were known as antigorite. He found that nearly all gave similar powder photographs, indicating a close structural relationship, but that there were noticeable differences on the basis of which the specimens could be separated into two groups, one giving a photograph similar to that of chrysotile asbestos, and the other containing the varieties called antigorite. Gruner (1937) came to similar conclusions, and in addition he indexed most of the reflections using the cell dimensions a 5.33, b 18.5, c 2 x 7.33 Å., β 93°, remarking in particular on the correspondence between 001 reflections in the two groups.

Qualitative studies of the optical properties of crystals under the polarizing microscope and refractive index measurements by immersion methods are made on slides placed upon the microscope stage. These procedures have the disadvantage that the crystals have a fixed orientation. Quantitative properties, such as optic axial angles, are measured on an accessory to the microscope, such as the universal stage, which although allowing rotation of the crystal necessitates auxiliary stereographic projection and is often considered laborious in operation and is not widely employed.

Physico-Chemical data clearly show that the presence of tridymite in a rock indicates the prevalence not only of high temperatures, but also of relatively low pressures during the formation of that rock. Whereas many authors (e.g. Shand, 1927; Larsen, 1936) have used tridymite as an indicator of temperature, the importance of tridymite as an indicator of pressure has been largely overlooked.

Although the presence of tridymite is by itself sufficient to enable the pressure, and hence the depth, at which a rock formed to be estimated within certain limits, the presence of tridymite in association with some other pressure-or temperature-indicating mineral allows much closer limits to be set.

Occurrence and relationships. Altered and garnetized gabbros occur in association with serpentinite occupying the lower south-eastern slope of Byne Hill, one mile to the south of Girvan. The rocks crop out above Balaclava Wood near the footpath from Bynehill Bridge to Craiglea. Although the precise form of the altered gabbros is not very well revealed, they appear to be dikes intrusive into the surrounding serpentinized harzburgite. Contacts with the serpentinite are exposed in two places but are evidently lines of later movement. The relationships of these altered gabbros to the main gabbro mass of Byne Hill are obscure. They are, however, probably contemporaneous, since sporadic garnetization of the main gabbro occurs along its contact with serpentinite farther to the south-west.

The hypersthene-gabbro of Centre 2, Ardnamurchan, is intruded largely amongst sedimentary rocks. Despite this fact the majority of the inclusions are, as described by Riehey and Thomas (1930), of granulitized basic and ultrabasic igneous rocks. Recently M. K. Wells (1951) studied this problem again, but was only able to find four occurrences in which the xenolith was undoubtedly of sedimentary origin. However, he considered in some detail a problematical group known as the banded granular hornfelses and concluded that, despite four possible modes of origin, the textural and compositional evidence favoured a development through the metasomatism of sediments.

In the course of an investigation on the determination of soil carbonates a sample of dolomite, from the Ibis dolomite quarry, Mr. Flinders, Queensland, was examined by X-ray diffraction and found to contain palygorskite. As this appears to be the first recognition of the occurrence of the mineral in Australia and as it has a number of commercial uses (e.g. as a drilling mud, catalyst, decolorizer, and absorbent), further work was undertaken.

During an investigation of the mineralization of the Leadhills- Wanlockhead lead and zinc mining district, rammelsbergite (NiAs2) was identified as an associate of niceolite (NiAs), reputed to have come from no. 4 level on the west branch of the New Glencrieff vein. The identification was confirmed on further material collected from the dumps at the Glenerieff mine, Wanlockhead, Dumfriesshire, and on specimens from the Scottish Mineral Collection (nos. 1952.5.18; 20; 21).