After a brief account of the work currently in progress on fine-grained sedimentary rocks in the U.S.S.R., and particularly in Azerbaijan, the concept of finely-dispersed minerals as against clay minerals is considered and some comments made on classification schemes. Conditions of formation and deposition determine whether sedimentation or diagenesis is predominant and hence the mineralogy of the deposit. Methods of mineralogical investigation are as yet ill-developed and much remains to be done. Finally, some of the wider problems in clay mineralogy are discussed and some suggestions made regarding the organization of suitable studies, particularly in the U.S.S.R.

Tonsteins, which are kaolinite-rich rocks with an amorphous groundmass containing much organic material, occur widely in European Coal Measure deposits. The nature of these rocks and their peculiar fabric is considered in detail and evidence is adduced which suggests that they were derived from soils and that the original organic components (micro-organisms and fungi) played a significant part in the formation of the kaolinite macrocrystals.

A study has been made of the mineralogical transformations that have accompanied the chemical weathering of some Palaeozoic clay-slates from New South Wales. The weathered sequences occur within a limited range of climatic and topographical conditions. Chlorite, interlayered with other expanded-lattice silicate minerals, is the least chemically stable of the primary constituents. Montmorillonite is a little more resistant to the percolating waters, whereas illite, present in all primary materials as the 2M1 polymorph, is relatively chemically stable. In the zone of maximum leaching, however, partial degradation and loss of illite is apparent. Kaolinite is the end-product of the breakdown of these clay minerals.

A study has been made of the relative abundance of clay minerals in about a hundred samples of sedimentary rocks of Pre-Cambrian, Cambrian, Upper Carboniferous, Permian, Triassic, Jurassic, Lower and Upper Cretaceous and Tertiary formations in the geosynclinal basin of the (Tethys) Himalayan zone. Chlorite predominates in the Cambrian sediments and illite in the Permo-Carboniferous to Tertiary sediments; kaolinite is a minor variable component and montmorillonite is scarce. The high-chlorite Cambrian sediments are rich in dolomite and gypsum and contain predominantly plagioclase felspars, whereas the high-illite Triassic and Eocene sediments are rich in alkali felspars, the dolomite is low and gypsum is absent. In the (newer) Tertiaries of the Middle Miocene age (fluviatile deposits in shallow freshwater basin) illite, kaolinite and montmorillonite are present in almost equal amounts but chlorite is undetectable. The predominance of kaolinite and montmorillonite in the sediments of the Lower Pleistocene period indicates the existence of a distinctly fluviatile facies in the basin of deposition. Interstratified illite-montmorillonite and montmorillonite are characteristic of Siwalik (Miocene) sediments and of Spiti-shale type Jurassic sediments.

Extensive deposits of red glacial drift and lacustrine clay occur along the east coast of Aberdeenshire and constitute the parent materials of important agricultural soils. The mineralogy of lacustrine clay at Tipperty, Aberdeenshire, has been determined by X-ray, differential thermal, and optical methods. General relationships between the mineralogy of this soil parent material and that of other red drifts in the vicinity are discussed with reference to the glaciology of the region. It is suggested that these red drifts were transported from the bed of the North Sea and not from Strathmore.

Chamosite has been identified as a regular member of the suite of minerals constituting the Lower Ecca shales in Natal. In this deposit, chalybite, Fe-chlorite and illite are all found to be secondary to the chamosite component which serves as the principal mineral host for iron, even in beds containing as little as 2·25% Fe. Aspects of composition, fissility and origin are discussed.

The specific surface areas of three montmorillonite samples, both alone and in admixture with kaolinite, have been measured. The results are discussed in relation to the possible existence of montmorillonite crystals.

Conflicting evidence has been published concerning the occurrence of an oxidation exothermic peak on differential thermal curves for NH4-saturated clay minerals. The present experiments show that this peak is affected by (a) the clay mineral concerned and (b) the nature of the thermocouple, Pt/PtRh thermocouples apparently acting as good catalysts for oxidation of released ammonia.