Accretionary lapilli have been described from rocks of all geological ages as well as from historical eruptions and in all cases they are associated with volcanic activity in which large amounts of steam accompany the eruption of dense clouds of ash material. Similar lapilli are described from deposits thought to have been deposited near to the flank of a well-established geosynclinal trough of the Precambrian Barama-Mazaruni Assemblage of British Guiana.

Chemical and mineralogical data are presented to demonstrate the overall similarity between granophyre and felsite from Rhum. However, although the compositions of the groundmasses of the two rock-types are very similar their characteristic textures are quite different. The felsitic texture is shown to be the result of devitrification of a glass, whilst the granophyric texture is attributed to the simultaneous crystallization of quartz and feldspar on the cotectic line in the system Ab–Or–SiO2–H2O. An explanation of the formation of the different textures is given in terms of the degree of super-cooling experienced by separate intrusions of the same magma in contrasted environments.

The determination of the stability conditions of chlorite, especially the stability temperature between 500° and 700° C. by B. W. Nelson, D. M. Roy, R. Roy, and H. S. Yoder complements the conclusions arrived at from the microscopical study of the interstitial chlorite in the diabase-minverite series of south-west England. These disprove the assumption that the chlorite generally originates from metasomatic reaction or devitrification of a glassy residue. Considering the formation of chlorite at a late stage of solidification, an explanation of its genesis is attempted on that basis. Possible effects on the petrologic evolution are discussed.

The chlorite taking part in the filling of amygdales and pseudoamygdales has formed preferentially by replacement of carbonates. Variation in form, degree, and associated products is exemplified. The appearance of a liquid carbonate phase, ascertained by P. J. Wyllie, O. F. Tuttle, and R. I. Harker in the system calcitewollastonite-water at temperatures between 640° and 680° C. seems to be realized likewise under natural conditions, and to provide a basis for the understanding of certain phenomena, especially in reaction zones.

According to the symmetry principle the fabric of a tectonite reflects the symmetry of the deforming movements. Where the symmetry of the deformation varies from place to place this should be reflected in a similar variation in the fabric. The possible variation in homogeneous strain is best shown by the range of variation possible in the deformation ellipsoid. It is argued in the paper that rocks commonly occur which show a variation in fabric from place to place similar to the whole or part of the possible variation in shape of the deformation ellipsoid. It is concluded that such rocks have deformed by homogeneous strain. This conclusion is supported by the shapes and orientations of pebbles in deformed conglomerates.

Local magnetic anomalies of large magnitude are found over the marginal ultrabasic rocks of the Loch Ailsh intrusion. An intrepretation of these anomalies based on the measured magnetic properties of the pyroxenites shows that they are not reconcilable with a low-angle structure as required by the hypothesis that the intrusion is a stratified laccolite, and suggests that they form a steeply dipping sheet at the syenite-limestone contact. Similar anomalies occur along the southern margin of the Loch Borralan complex, suggesting steeply dipping marginal ultrabasic rocks.

A microscopic study of sandstones of Old Red Sandstone and Carboniferous ages along a part of the North Crop of the South Wales Coalfield reveals that the rocks are extensively fractured. The fractures occur in the quartz grains and are often filled with bubble-like inclusions. These planes of inclusion were studied in specimens over a wide area, and are related to the directions of faulting and jointing. The consistency in trend between the macro and micro-structures suggests a homogeneity of the deforming stresses.

Quartz keratophyre sills interbedded with folded Devonian sediments in the Annan River Tinfield area in North Queensland have probably been intruded when the sediments were unconsolidated or only partly consolidated. The sills are compared with similar occurrences in Wales, Alaska and Australia. Some of these may have been extruded prior to burrowing into sea-bottom muds but the North Queensland examples are probably true, though very high-level, sills.