In the first part of this paper (33) the effects of heat on the potash-soda-felspars were considered, and a possible explanation in terms of atomic rearrangement was proposed. In this part the experiments, and deductions from them, will be applied to some questions of petrogenesis and differentiation, particularly in connexion with the granites and granite-pegmatites.

In 1824 Henry Witham discovered a beautiful red epidote mineral in the lavas of Glen Coe, Argllshire, which was described by Brewster under the name withamite. An analysis was made by Coverdale of obviously highly impure material, but later a more complete one was carried out by Heddle on carefully hand-picked mineral. Heddle admits, however, that his analysis shows ‘by no means a satisfactory agreement with the composition of epidote’, and from his figures it would seem very likely that his analysed material was also impure. In the Glen Coe Geological Survey Memoir no further data were recorded of this mineral, Heddle's analysis and description merely being quoted.

The writer visited Glen Coe and collected material in a cutting on the new road, approximately 5½ miles west of King's House (Geological Survey, sheet 53), where it is fairly well exposed. It occurs in a fine-grained, rather altered andesitic rock, as narrow veinlets up to 120 mm. in length and 50 mm. in width ; also as infillings of vesicles. The veinlets may be straight or highly irregular; in the former case they possibly follow joint-planes, for the rock will often fracture along such a plane, producing one or two surfaces covered with the red epidote mineral. The vesicles vary greatly in size from approximately 0·3 to 70 mm., and in the larger ones calcite is a common associate.

During the course of a survey of the mineral deposits of the area immediately south of the Murchison Range in the NE. Transvaal, a visit was paid to the Union mica mine, one of the only two producing mica mines in South Africa. Interest very soon centred round the occurrence of flattened garnet inclusions in a large number of the mica sheets inspected in the trimming hut. A description has been given by A. L. Hall of the nature of the occurrence of the muscovite, while the geology of the area surrounding the mine is shown in a general way on his map. However, no mention was made of platy garnet crystals; and the Union mine is shown on the map as the Premier mica mines, a name by which it was then known. It is situated on the farm Inyoko (or Inyoka) (24° 8' 25" S., 30° 50' 49" E.) on the western slopes of a ridge overlooking the Sandspruit valley, and lies only two miles from Mica Siding on the Selati Railway, though best approached from Gravelotte by road, a distance about 22 miles.

The sedimentary iron ores of Britain all contain small amounts of phosphorus which is generally present in the colloform mineral collophane. In a rather uncommon class of oolitic ironstones recently described from the Yorkshire coalfield, a crystalline phosphate was found which has now been identified as francolite and forms the subject of this paper. These oolitic ironstones occur in the immediate roofs of coal seams, are of freshwater origin, and consist of ooliths of kaolinite and isotropic clay in a groundmass of chalybite. During the later stages of their diagenesis, francolite, associated with quartz, calcite, pyrite and blonde, and much kaolinite, crystallized within the ooliths (fig. 1) and in the shrinkage cracks (fig. 2) of the ironstone, and sometimes replaced the ooliths and groundmass. The francolite is present in all of the three oolitic ironstone occurrences described from Yorkshire, and also in the similar rock from Derbyshire in the Geological Survey's collection of iron ores. Details of these localities are given in the ironstones paper. The writer has also found similar francolite in an ironstone nodule from a parting in the Mühlenbach coal seam of south Limburg, Netherlands. Although not recorded, it may be expected to occur in some of the black-band ironstones and other rocks closely associated with coal seams. The following account is based entirely on material from Robin Hood quarry, Thorpe-on-the-Hill, 4 miles south of Leeds.

According to their atomic arrangement the minerals found in clays can be divided into the two-layer and the three-layer structures. The members of the first group have alternating silica and alumina layers, e.g. kaolinite; the members of the second group have one alumina (or magnesium or iron) layer between two silica layers, e.g. pyrophyllite. The clay minerals can also be classified into two groups according to their power to diffract X-rays. The first group gives good powder photographs with twenty to forty well-developed lines if copper or iron radiations are used, whereas the second group gives poor diagrams with only a few lines, usually under fifteen, some of which are wide and indistinct. Exposure times for the second group are about two to three times as large as those for the first group. The poor diffracting power for X-rays may be due either to a lack of regularity (‘roughness’) of the lattice, or to an extremely small size of the single crystallites. It is possible that both effects are aetive to some extent, but the lack of regularity of the lattice is probably the main reason. Neither the microscopic evidence nor sedimentation data based on Stokes's law show these minerals to have particles smaller than 10-6 cm., though it is possible that the optical effects such as double refraction are due to aggregation, and it is clear that the equivalent radius is an unsatisfactory measure for plate~shaped particles. Both classifications are shown on p. 141.