During a recent investigation of the hydrothermal mineralization associated with the Shap granite, nacrite was found in a quartz vein in the Shap Blue quarry. This quarry has been opened for roadstone in the metamorphosed Borrowdale lavas, 700 yards north of the granite contact on the west side of the Penrith-Kendal road, three miles south of Shap village. Further examination showed that nacrite often coats shear joints in fault breccias and shatter belts in the metamorphosed andesites. It is associated with chlorite, haematite, pyrite, and traces of erythrite, and often with later quartz, calcite, and baryte veins.

Nacrite from this locality occurs as microcrystalline aggregates on joint faces, usually only in thin layers but layers up to 5 mm. thick have been found.

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.

Jurupaite was discovered at Crestmore, California, by A. S. Eakle in 1921. The mineral was found in a quarry which was rapidly being enlarged, and Eakle stated that it was probably represented only by the one specimen which he had collected. He showed that it was a hydrated calcium silicate containing magnesia, with the composition 2(Ca,Mg)O. 2SiO2. H2O, the ratio of lime to magnesia being approximately 7 : 1.

This specimen passed into the keeping of Professor A. Pabst, who kindly made a portion available to the writer. He confirmed that it was unlikely that any other specimen existed. The jurupaite consisted of rosettes of white needles or fibres, about a centimetre in diameter. A brown discoloration was observed on the exposed outer surfaces of the specimen, but not on freshly cut surfaces. Calcite was present in contact with the jurupaite.

A widely used method of quantitative analysis in clay mineralogy . is the comparison of the intensities of X-ray reflections from a socalled standard mineral with the unknown. In the clay minerals where isomorphous substitution is frequent the term 'mineral' generally covers a range of compositions and what is meant by such a term is a group of closely similar minerals. The so-called standard mineral is any member of a group which the particular worker may have in a pure or almost pure condition. If the standard mineral and the mineral in the unknown give different intensities for an equal volume of material the results will be inaccurate.

Examination in reflected polarized light is now widely used in metallography as well as for mineral sections. The recent introduction of graded diamond abrasives has overcome most of the difficulties in polishing that have previously hindered the widespread use of this important method.

The present instrument (fig. 1) has been designed to give the best available control over the conditions of illumination and extinction, and to provide for the complete analysis of the reflected elliptically-polarized beam. A robust stand of normal grade suitable for general research purposes and works control is provided with a stage rack, reflector unit, and the usual polarizing equipment. It may be adapted for transmitted light by the addition of the detachable substage shown on the right of the figure.

Recent investigations into the mineralogical constitution of fireclays have revealed the presence of a micaceous mineral in all samples examined. Naturally-occurring clays may contain up to 30 % of this type of mineral, and exceptional concentrations have been found in the Yorkshire area. Tests on the purified mineral from this source have proved it to be a hydrous mica of an unusually large grain,size.

The narrow belt of granophyre lying to the west of Blaven in the Isle of Skye showed so admirably the effects of chilling that a series of specimens was collected by one of us (L.R.W.) and found to contain, in the chilled marginal rock, phenoerysts of tridymite, now inverted to quartz. In the Thulean Tertiary igneous province former tridymite is known from certain acid lavas, for example, the Tardree rhyolite (von Lasaulx, 1877) and certain Icelandic liparites (Hawkes, 1916), and from metamorphosed arkoses adjacent to basic igneous intrusions (Harker, 1908, 1932), but it has not previously been noted in the intrusive acid rocks. In addition to phenocrysts of tridymite inverted to quartz, there is present in the groundmass of the unchilled granophyre a second generation of inverted tridymite crystals, surrounded by a final stage of quartz and felspar which has crystallized with normal microgranitic textures. Some of the textural features resemble those of the normal Skye granophyres, while others resemble certain metamorphosed arkoses.

In May 1950 the writer received from Mr. Henry Burgess, of Spiggie, Shetland Isles, a specimen of a material which had been found by a local quarryman. The property of the material which had particularly attracted attention was that when it absorbed water it became soft and pliable and looked like wet leather. The specimen, measuring approximately 4x2X1 inches, had a brownish iron-stained outer skin, and showed slight signs of foliation parallel to its length. Internally it consisted of a white to greyish-white compact material, in which were embedded portions of the syenitie country-rock. This white material has been shown to consist dominantly of the mineral palygorskite (attapulgite).

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.

In a previous paper (Brindley, 1951) chamosites from a number of localities were examined, and their structure was shown to be of kaolin-type. Furthermore, it was shown that many of the variations in X-ray powder diagrams of chamosites can be accounted for by the existence of two kaolin-type structures, one hexagonal and one monoclinic, with simply related ceil dimensions.

Many of the Dalradian and 'Moine' rocks of Donegal, and the Caledonian granites associated with them, have been shown to bear structural and petrographic resemblance to the rocks of similar age in Scotland. In Scotland riebeckite-bearing minor intrusions occur at many localities and include Ordovician, Caledonian, Carboniferous, and Tertiary intrusions. There appears to be no record of analogous rocks containing sodic amphiboles occurring in situ in NW. Ireland. The purpose of this paper is to record the existence of two small intrusions of such rocks in western Cloghaneely, Co. Donegal.

For the examination of grains with high birefringence the usual types of wedge or plate often fail to give clear indications of optical sign. In such cases plates up to the 20th order on Newton's scale (relative retardation about 10μ) may be employed.

Determination of sign of length of a crystal—A detrital crystal of zircon (length positive, i.e. slow) about 300-400μ thick observed between crossed nicels in conjunction with a 15-20th-order plate will illustrate this test. When the length of the crystal is parallel to the fast direction of the plate (fig. 1 A) there is compensation, and colour bands are seen. Without the plate, or with the crystal rotated through 90° (fig. 1 B), only high-order whites are seen. A crystal of high birefringence with length negative (e.g. a bipyramid of anatase) will give colour bands in position B, and none in position A.

The concept of a space lattice is fundamental in crystallography. A space lattice, or more simply a lattice, is an indefinitely extended regular array of points, in which the environment of each point is exactly the same. This implies that if the lattice is (1) translated parallel to itself, or (2) inverted about one of its points, or (3) rotated it will remain precisely the same as it was before such transformations. Such projective transformations as translation, inversion, or rotation are called symmetry operations. A lattice therefore may be said to be invariant in respect to certain specific symmetry operations. It is then said to possess certain symmetry elements.

Available information on the clay mineral saponite shows an amazing degree of variation and considerable discrepancies, probably due to investigation of impure samples. The clay mineral described in this paper was essentially pure and furthermore it seems to represent a magnesium end-member of the montmorillonite group.

Optical, chemical, X-ray, and differential thermal data of this particular saponite are presented, as well as data on some associated minerals, namely, a chlorite, two antigorites, and a chrysotile-asbestos.

The influence of Mg++, as the only natural occurring exchangeable base, on the nature of the X-ray diffraction and differential thermal diagrams leads to some interesting observations.

The instrument is designed for the study and measurement of the optical characters of crystals and anisotropic materials. The conoscopic phenomena, over a wide area for any selected orientation of the specimen, may be brought into view on a hemispherical screen, without the use of a microscope or lens system. The parts of the interference figure retain their true angular relationships, directly displaying for observation or measurement values of 2V for any angular separation of the optic axes.

In a recent note in this Magazine, Dr. R. B. Elliott points out that inclined contacts of grains of considerably different index may bias the result of a micrometric analysis. That there may be some bias of this sort is hardly to be doubted, though I am inclined to feel that he greatly overestimates its importance. His demonstration, however, involves a fallacy so long and widely held as to be worth refuting.