The Cork Red Marble is a coarse, graded but poorly sorted, re-sedimented lime-conglomerate, of Caninian age. It occupies a central position within 4,000 feet of Waulsortian limestones. The pebble content of “porcellanous” calcite mudstone—not in reef facies—is set in a matrix of red clay. Mixed biofacies are represented, because of reworking: fragmented large molluscs, indicative of a nourishing reef habitat, contrast forcibly with a mollusc spat-ostracod assemblage as found in the pebbles. The conglomerate was probably formed when back-reef sediments, which included red clay, were elevated and then redeposited, possibly by turbidity currents

Limestones consisting of recrystallized shells of lamellibranchs, originally composed of aragonite, contain a brown-coloured calcite which is strongly pleochroic. Maximum “absorption’ corresponds to the extraordinary ray. The effect is shown to be a pseudo-pleochroism, caused by scattering of the extraordinary ray by inclusions, which follow the shell structure, in the calcite. Aragonite shells, which have not recrystallized, show a pseudo-pleochroism of similar colour but reversed sign. The inclusions originate in the conchiolin of the lamellibranch shell; their present nature is not known. Their progressive destruction by thermal metamorphism is described.

The Mt. Alford ring-complex consists of a central boss of porphyritic microdiorite, granophyre and andesite, with a semicircular zone of rhyolitic and trachytic ring-dykes intrusive into upturned sandstones, and cross-cutting non-arcuate dykes of rhyolite, comendite, trachyte, andesite, phonolite and basalt. The petrology and geochemistry of these rocks are discussed and it is concluded that the various rocks have been produced by differentiation of a basic magma.

Conjugate shear fold systems are often developed when brittle laminated rocks are subjected to stress. These structures relationship of the principal axes of stress to the surface being folded. Within first order shear zones conjugate second order shear folds may be developed as a result of reorientation of the stress axes in these zones.

The axes of both first and second order conjugate shear folds may form at any angle to the principal stress axes and to the direction of movement within the shear zones. The principal axes of stress are most reliably determined from the orientation of the axial planes of the first order conjugate folds.

The paper describes those features of the Scardroy Lewisian mass, a sheet-like body of basement rocks thrust into the younger Moine Series, which suggest that this mass was driven up through a thickness of several kilometres of Moinian rocks and inserted as a narrow wedge within the Moine succession, never penetrating the highest members of that succession. The relations of the Scardroy mass, to which most of the Lewisian outcrops of central Ross-shire belong, differ from those of the more westerly outcrops of Lewisian in the region of Glenelg where the basement rocks have been shown by the work of Clough and others to represent wedges lying between tightly-compressed synclines of Moinian rocks which form a more or less autochthonous cover. The connection between the two types of structure is discussed.

A brief description is given of the sedimentary features of the Aberystwyth Grits. The form and distribution of the major folds are demonstrated and their relationship to the sedimentary pattern is indicated. Thrust, wrench, and normal fault systems are discussed in relation to the stress field which initiated them. Cleavage and joint patterns are also considered. Finally the major structures of the Aberystwyth Grits are compared and contrasted with those which have developed in adjacent areas.

Bowen's petrogenetic grid is a PT projection containing univariant curves for decarbonation, dehydration, and solid-solid reactions, with vapour pressure (Pf) equal to total pressure (Ps). Analysis of experimental data in the system MgO–CO2–H2O leads to an expansion of this grid. Three of the important variables in metamorphism when Pf = Ps are P, T, and variation of the pore fluid composition between H2O and CO2. These can be illustrated in a three-dimensional petrogenetic model; one face is a PT plane for reactions occurring with pure H2O, and the opposite face is a similar plane for reactions with pure CO2; these are separated by an axis for pore fluid composition varying between H2O and CO2. Superposition of the PT faces of the model provides the petrogenetic grid. The reactions within the model are represented by divariant surfaces, which may meet along univariant lines. For dissociation reactions, the surfaces curve towards lower temperatures as the proportion of non-reacting volatile increases, and solid-solid reaction surfaces are parallel to the vapour composition axis and perpendicular to the PT axes. The relative temperatures of reactions and the lines of intersections of the surfaces can be illustrated in isobaric sections. Isobaric sections are used to illustrate reactions proceeding at constant pressure with (1) pore fluid composition remaining constant during the reaction, with temperature increasing (2) pore fluid composition changing during the reaction, with temperature increasing, and (3) pore fluid changing composition at constant temperature. The petrogenetic model provides a convenient framework for a wide range of experimental data.