Among a series of specimens collected by Mr. H. G. Dines from the South Crofty mine, Illogan, one was found to contain bertrandite and beryl. The specimen was obtained from the back of the 290-fathom level, east of Robinson's shaft, in the no. 1 lode, which hades north and at this position is 3 feet 10 inches wide. The location of the specimen was about 9 inches from the granite footwall. In hand-specimen the rock is a normal stanniferous veinstone composed of massive quartz tinted grey-green by disseminated chlorite and tourmaline and traversed by stripes of blackish ‘peach’ and white vein quartz which run parallel to the lode walls. Microscopically it is composed essentially of quartz, chlorite, and tourmaline, with subordinate orthoclase, fluorspar, and cassiterite, accessory apatite, and local bertrandite and beryl. In its texture there is evidence of an early formation of stanniferous chlorite-tourmaline peach with quartz-fluorspar gangue, which was subsequently fractured and, later, more or less completely healed by an accession of quartz, some redistribution of the earlier minerals having taken place at the same time. The place of bertrandite and beryl in this general scheme is discussed below (p. 576).

Direct-reading refractometers depend upon observation of the boundary between transmission and total internal reflection, when a flat surface of a specimen is placed in optical contact with the flat surface of the isotropic medium employed in the instrument. This medium is usually a heavy lead glass in the form of a hemisphere or prism. The upper limit to the range of indices which can be measured is determined by two factors: the refractive index of the medium employed in the instrument, and the refractive index of the liquid used to make optical contact between the two surfaces.

Liquids of high refractive index are essential to such instruments, and some reference to them may be conveniently made at this stage. Various pure liquids, mixtures, and melts of high refractivity have been proposed from time to time, chiefly as immersion media.

Many attempts have been made to discover the causes of the various colours displayed by minerals. If we except fluorescence and allied phenomena, the diiteren~ theories advanced may in general be grouped under one or other of the following classes:

1. (1) Colour may be due to an intrinsic property of atoms, ions, or molecules. Examples are copper atoms, chromate ions, and ferric oxide molecules.

2. (2) Colour is sometimes attributed to structural causes. The blue tints of certain halites have been explained in this way; but a fresh suggestion is made in this communication.

3. (3) A pigmentary impurity may cause the observed colour. The colour of the ruby is believed to be caused by the presence of a red chromic oxide. The blue tint of a Yorkshire aragonite is shown later to be due to a copper salt.

4. (4) Some minerals show plays of colours due to interference and/or grating effects. Labradorite and opal are cases in point. Into this group also fall minerals like bomite which become iridescent from tarnish.

Jamesonite is described in Dana (1892, p. 122) as a soft, brittle, steel-grey to lead-grey mineral forming acicular orthorhombic crystals, a: b = ‘0·915:1’ (in error for 0·8195:1), with perfect basal cleavage; composition, 2PbS.Sb2S3; principal occurrence, Cornwall. Jamesonite is taken as the type member of a group that includes cosalite, described in the first of these studies (1939), dufrenoysite, and other minerals of analogous composition.

Early this year Mr. Arthur Russell brought me a small specimen of ore from Mill Close mine, Wensley, Derbyshire, for identification. An X-ray powder photograph p1. xxviii, fig. 3, showed that the mineral has the pyrite structure with unit-cell edge a 5.49 Å., and a chemical analysis of a small sample of specific gravity 4·82, yielded Ni 16·69, Fe 29·30, S 53.40% (see table 1). These data identified the specimen as bravoite, a rare mineral new to the British Isles. At a later date Professor W. G. Fearnsides interested himself in our preliminary work and kindly provided me with further specimens of bravoite and accessory minerals from the Mill Close mine. These also have been investigated.

In December 1933 Major W. A. Pover, who was then ‘Iraq Public Works Department Executive Engineer of the Mosul and Kirkuk Liwas, showed the writer a portion of a meteoritic stone. This had been in his office doing duty as a paper-weight since it was brought to him by two of his men shortly after its fall.

At that time Major Pover was in camp, supervising the erection of the permanent steel road bridge that spans the river valley some five kilometres upstream of Tauq village. His men told him that the meteorite fell in the middle of the night; it was likened by those who saw it to the moon falling, and people were awakened by the jolt of its fall. When the morning came, some of his men went out to seek it, and they in fact found it. They described it as about two and a half feet in diameter, and they broke it up with a sledge hammer, when the fragments were eagerly collected by those who happened to be present. A portion they themselves took to Major Pover.