Morphological features of brittle fracture in glass, ceramics and rocks include fracture origin, mirror plane, mist, hackle, radial striations and concentric undulations. The plume structure which characterizes many joint surfaces is analogous to striations or ‘grainy steps’ developed on fracture surfaces of ceramics. Joint surfaces with plumose structures result essentially from tensile failure, but may also be affected by shear stresses. The ellipticity of the plumose structure commonly observed reflects a region of secondary tensional strain parallel to bedding developed due to anisotropic elastic properties of the rock as a result of formational loading. Striations and hackles may have similar orientations. A distinction between them is important since hackles on a joint surface indicate a rapid fracture whereas striations can result from a slow fracture. Intense undulations without hackling may indicate a slow fracture. A smooth mirror plane on a joint surface indicates that while the fracture had been formed, stresses had reached a certain maximum and then subsided. On the other hand, a joint surface that shows a mirror plane with hackle around its boundaries indicates that no decrease of stresses occurred during fracture, and fracture was possibly produced under a constant stress. Since AH is a material property, the fracture stress σfresponsible for a rapid fracture may be calculated following a determination of the mirror constant AH of the rock by σfrH½=AH

Whitcliffian and Downtonian macroplants of rhyniophyte morphology are described from Capel Horeb Quarry, Powys, Wales. Both floras contain Hostinetla Stur, Cooksonia Lang and Steganotheca Edwards. They are briefly compared with others of similar age, and the contribution, particularly of the Whitcliffian assemblage, to our knowledge on the early colonisation of the land by vascular plants is discussed.

Sheath and core structure results from secondary hydration and devitrification, initiated at relatively high temperatures during cooling, of the intrusive pitchstone sheets. Less than 0.5% of the water present is an original magma component.

A thin-section study of the orientation of long-axes of quartz grains was conducted for a similar type fold of overturned cross-bedding in Carboniferous sandstones. Preferred orientation of detrital grains of the fold shows that (a) apparent long-axes of grains are parallel to the axial plane in a plane normal to the fold axis, (b) long-axes of grains in the axial plane are oriented in a high angle to the fold axis, and (c) grains are aligned parallel to the fold axis in a plane perpendicular to the other two directions. From the dimensional orientation of quartz grains in this sample, it is concluded that the overturned fold was made by a shear mechanism immediately following sedimentation in which the original quartz grains were reorientated by small-scale slippage along the shear planes.

Thirty-seven new K-Ar isotopic ages are reported from the Isle of Man. Thirty clay mineral concentrates from vein gouges and altered wallrocks when interpreted with the remaining seven whole rocks and mineral ages indicate that at least five periods of metasomatism have affected the rocks. It is argued that these can be identified with episodes of ore deposition. The earliest phase, N of the Foxdale area, occurred at 310–320 Ma and appears to be the most extensive of the episodes recorded. In the S of the island, mineralization occurred later at c. 285 Ma and again at c. 250 Ma. Minor Triassic activity is expressed by a cross-course on the Foxdale lode, and a single locality displaying Tertiary mineralization was recognized. The conformity of these ages to the general episodic pattern of ore deposition in the British Isles is demonstrated.

Several conodont faunas from the Ilfracombe Slates are described and their age discussed. One is referred to the conodont zone of Polygnathus varcus (late Givetian-early Frasnian). The conodonts, the first from the Devonian of North Devon, indicate that, in terms of the German standard, the uppermost Ilfracombe Slates are no younger than basal Manticoceras Stufe in age. The position of the Middle/Upper Devonian boundary within the North Devon sequence is discussed with reference to the limestones of the Combe Martin Slates (the ‘Ilfracombe limestones’), which are regarded here as Middle Devonian in age.

Paired eruptive complexes consisting of a symmetrical and an asymmetrical cinder cone are described from Mount Etna. The contrasts in shape and fumarolic activity and the temporal relations of the cones are ascribed to the level of magma in the underlying conduit.

The pahoehoe and pillow lavas described show many common features of morphology. A finger-like or digital flow mechanism adequately accounts for the similarities although the growth mechanism of individual lobes is by markedly different processes. An important distinction is the time taken to develop a rigid crust and its subsequent expansion by fracturing and auto intrusion. This is rapidly achieved in pillow lavas but only occurs at a late stage in the pahoehoe forms. The main factors controlling the formation of these features are the environment of eruption, the volume of erupted material and the rate of eruption.