Two groups of mafic dikes occur within a terrain of metasomatic granitic rocks and quartzofeldspathic gneiss in the central Bighorn Mountains of Wyoming. The younger group consists of dolerite dikes which possess ophitic, subophitic, hypidio morphic-granular and microporphyritic textures. Mineralogically, the dolerites consist primarily of plagioclase (average An 54), augite (average Ca: Mg: Fe = 41: 43: 16) and uralite. The older dikes are metadolerites which possess granoblastic margins that grade into interiors characterized by relict subophitic and locally, porphyriti texture. Dike margins consist of fresh plagioclase and hornblende primarily. Interiors are principally clouded plagioclase (average An 46), augite (average Ca: Mg: Fe = 39: 54: 7) and uralite and/or hornblende. Textural, mineralogic and chemical data indicate that the meta-dolerites were derived from an earlier generation of mafic dikes comparable to the unmeta-morphosed dolerites in the area. The dikes were metamorphosed under low amphibolite facies conditions during the last regional metamorphic event which affected the country rock. During their metamorphism, minimal amounts of water from the country rock facilitated the total recrystallization of the metadolerite margins and the complete or nearly complete replacement of pyroxene by hornblende in the margins. In general, dike interiors underwent no textural changes other than partial destruction of the original subophitic plagioclase–pyroxene relationship as the pyroxene was replaced peripherally by hornblende. This process was accompanied by diffusion of calcium from the plagioclase to form the hornblende, and by diffusion of iron from the augite to cause the clouding of adjacent plagioclase.

The history of the study of homological variation in the archaeocyathans is reviewed and some morphological details defined. Homological variability is considered at generic and familial level and a new detailed table of the homological series at generic level is presented.

Lenticular compositional layering and/or rodding in blastomylonitic rocks, in the simplest situations, can be controlled by (i) the presence of more than one mineral in the parent rock and (ii) relative large grain-size of the parent rock. Deformation and recrystallization during non-cataclastic ductile deformation can produce compositional layering, without chemical segregation.

Evidence from a series of boreholes in the Shoreham area demonstrates the occurrence of an outcrop of Woolwich and Reading Beds lying beneath the drift deposits which form the floor of Shoreham Harbour. The classification of the Eocene strata, their structural relation to other deposits in the area and their conditions of deposition are examined.

The Katete carbonatite is situated in an area of rifting in the NW of Rhodesia. It consists of varieties of carbonatite and of agglomerates. There is little alteration of the surrounding sandstones and there are no associated alkaline rocks. The age of the complex is mid-Cretaceous, which is about the same as the later stages of carbonatite activity in the Rufunsa Valley in Zambia.

Elucidation of the effects of secondary alteration on the stromatoporoid skeleton is important for the development of a sound taxonomy of the group. Four stages of progressive alteration in Palaeozoic stromatoporoids are identified and their effects summarized.

Four volcanic rocks from northern Tunisia and a syntectonic granite from northeastern Algeria were dated by the K—Ar method. The stratigraphic position of the dated rocks is determined mainly by their relationship to biostratigraphically calibrated Miocene tectonism. The K—Ar age of the granite, which is younger than latest Burdigalian and older than Early Tortonian, is 16.8 m.y. (*15.8 m.y. calculated with western decay constants). Rhyodacite at 10.9 (*10.4), rhyolite at 8.8 (*8.3), basalt at 7.3 (*6.9) and adesite at 7.0 (*6.7) m.y. are all apparently younger than Lower Tortonian.

Examination of layered rocks from the Currywongaun-Doughruagh Intrusion, Connemara, Eire, has suggested a possible connexion between adcumulate crystallization and the formation of thin, yet very persistent igneous layering. It is thought that as a result of adcumulate crystallization proceeding under undisturbed conditions near the top of a pile of cumulus crystals, a zone of liquid, supersaturated in those components not required for adcumulate growth, could be generated in the magma just above the cumulate pile. Crystallization from such a supersaturated zone would result in the cumulate phases, inter-directly beneath. The rhythmic deposition is lacking, could by this mechanism build up a sequence of cumulate layers interbedded with fine-scale layers of a non-depositional origin.