A Green mineral akin to monazite, derived from the State of Travancore in southern India, was investigated chemically during the first world war by the Imperial Institute, London, and was found to be exceptionally rich in thorium and uranium. No description of this unusual 'monazite' has hitherto been published.

The high content of radioactive elements in this mineral led the State Geologist, Mr. Venkitarama Mahadevan, to have a deposit opened up in 1945, in readiness for an assessment of the radioactive mineral resources of Travancore. At the invitation of the former Dewan, Sir C. P. Ramaswami Aiyar, K.C.S.I., K.C.I.E., this assessment was carried out by Dr. C. F. Davidson early in 1946.

During an investigation of stibiconite and cervantite (Vitaliano and Mason, 1952) we examined many specimens of antimony ochre. The commonest mineral in these specimens proved to be stibiconite, but others were found, and to establish their identity it became necessary to study the secondary products as a group. The mineralogy of these oxygenated secondary antimony minerals presents unusual problems on account of their earthy and often inhomogeneous nature. Our experience indicates that without X-ray powder photographs it is difficult to establish the true identity of much of this material. The purpose of this paper is to present the results of our investigations and to provide more adequate descriptions of some little-known species.

The bluish-green epidote-like mineral, pumpellyite, was first described by Palache and Vassar (1925) from the copper-bearing amygdaloidal lavas of the Lake Superior region. It has since been reported from many parts of the world in rocks which have undergone low grades of metamorphism, especially types such as amygdaloidal spilites and diabases (Burbank, 1927; Waldmann, 1934; Tsuboi, 1936; Quitzow, 1935, 1936; Klein, 1939; Laeroix, 1942; Seether, 1942; Zavaritsky, 1944:; Korzhinsky, ]944; de Roever, 1947a, b; Kvasha, 1950) and in glaueophane-schists and even eclogites in which pumpellyite is often associated with lawsonite (Irving, Vonsen, and Gonyer, 1932; Quitzow, 1935, 1936 ; Lacroix, 1942 ; Switzer, 1945 ; de Roever, 1947a, b, 1950). W. P. de Roever (1947a, 1950) has suggested a pumpellyitic facies of metamorphism as a lower-grade equivalent of his lawsoniteglaucophane subfacies, characterized by high confining pressure, but subordinate shearing stress and thermal influence, and passing into unmetamorphosed rocks.

The minerals to be described were found by Dr. Duncan M. Morison, of Edinburgh, and the writer in a small quarry having a national grid reference of 29/076184, which is at present being excavated for road surfacing material. The quarry is situated on the east side of the coast road between Lochinver and Ullapool at a point ¼ mile south-west of the southern margin of Loch an Arbhair (called Loch a' Choin on early editions of the Ordnance Survey sheet 101) and ¾ mile south-west of the bridge carrying the Lochinver-Ullapool road over the river Kirkaig, the course of which here marks the boundary between the counties of Sutherland and Ross.

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.

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.

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.

Hillebrandite was first characterized by Wright as a white, fibrous mineral with the composition 2CaO.SiO2.H2O, optical properties αNa 1·605±0.005, γNa 1·612±0.003, moderately weak birefringence, parallel extinction, optic axial angle 2E between 60 and 80° and density 2.692. X-ray powder data were first published by Vigfusson, but these have not been substantiated by later investigators (Clark and Bunn, McMurdie and Flint, and Heller and Taylor). Synthetic dicalcium silicate β-hydrate has been found to resemble hillebrandite closely.