In addition to meeting various geophysical requirements, an ultrabasic upper mantle must provide a source both for olivine nodules and for the range of “primary” basaltic types, i.e. those which are apparently uncontaminated and undifferentiated. Potassium abundances and other considerations suggest that olivine nodules represent upper mantle which has previously yielded basalt by partial fusion but is now “barren”, principally because of potassium depletion, and is unable to yield more. The possible range of potassium contents of “parent” upper mantle (still able to yield basalt) is considered and it is argued that at any concentration within this range, the potassium is not distributed between the main mantle phases as a trace component but is contained largely within a potassium-rich minor phase. It is shown that this phase is probably an amphibole and may make up between 2 and 20 wt. per cent of parent upper mantle. In addition to the isochemical phase changes to be expected with increasing depth, a transition is proposed from an upper zone of amphibole-free (barren) upper mantle to a lower zone of amphibole-bearing (parent) upper mantle. The petrogenetic and geophysical implications of such a model are discussed; in particular it provides a possible explanation for the Gutenberg low-velocity zone.

Previous research by the present author has established the Caledonian tectonic history of the Manx Slate Series. In particular three episodes of folding (F1, F2, F3) were recognized. The present contribution describes the sequence of metamorphic textures impressed on these metasediments during this polyphase deformation.

Two distinct episodes of metamorphism have affected the Manx Slate Series. A syn-tectonic F1 metamorphism produced a chloritezone alteration throughout the Island. A later F2 metamorphism was primarily responsible for the growth of small porphyroblasts of several minerals within the pre-existing F1 fabric. This second metamorphism is confined to the hinge of the large-scale F2 Manx synform and its effects are visible in an elongate belt following the middle of the Island. The emplacement of the porphyroblasts began during the F2 movement and continued into the static interval which preceded the third (F3) movement-phase. It was completed prior to the F3 deformation.

An intrusion associated with the Ratagan Complex (Nicholls, 1950) outcrops in Glen Lichd, east of the main omplex. It is separated from the main complex by the Strathconon fault. The intrusion consists of red medium-grained leucocratic granodiorite and adamellite.

A new ostracod from the London Clay (Lower Eocene) of the London Basin is described as Paijenborchella geoffreyi sp. nov. and compared with other species of the genus.

A foraminiferal fauna in the Lower Permian Marls of the Lowpit Lane Borehole, Derbyshire, included numerous examples of a new species of the genus Aschemonella Brady. This is described as Aschemonella longicaudata sp. nov. and the possible growth form of the genus is discussed.

A study of the geological history of a small dioritic mass near Gairloch has revealed the presence of rock types and structural elements not represented in most of the Lewisian of the district. Coarse grained diorite, showing intrusive relations to granulitic rocks of possible sedimentary derivation, has been foliated and subsequently intruded by microdiorite during pre-Laxfordian episodes. The Little Sand mass has subsequently been involved, probably as a thrust slice of basement, in the sequence of Laxfordian (about 1,500 m.y.) structural and metamorphic episodes which affected the more common Lewisian metasediments and metabasites.

It is established that the Dawros peridotite of north Connemara contains basic plagioclase mainly in a gabbroic layer in the peridotite but occasionally as rare crystals in the peridotite itself. Carlsbadalbite twin extinction angles indicate that the composition of this feldspar is similar to that in the adjoining Currywongaun-Doughruagh intrusion and that in the ultrabasic rocks of the Cashel-Lough Wheelaun intrusion and the Roundstone intrusion of south Connemara. As all these feldspars are extremely heavily saussuritized because of metamorphism it is difficult to determine their precise composition and so an exceptionally fresh plagioclase from an anorthosite in the Currywongaun-Doughruagh intrusion has been chemically analysed. It is An91·5Ab6·8Or1·7. This is in close agreement with the composition given by measuring 2θ(131)–2θ(131), the refractive indices and 2V while the extinction angles of Carlsbadalbite twins give only a slightly too anorthitic result (An95). The particularly calcic nature of the plagioclase is strong evidence that all the ultrabasic bodies in Connemara are genetically related. Some new structural ideas on the Dawros body are suggested which integrate the structure, petrography and cryptic variation.

Chemical and modal analyses have been made for a series of ten homogeneous specimens of meta-sediments, with identical metamorphic histories, from the aureôle of the Ardara pluton. Results showed that total amount of garnet was determined by the quanity of “free ferrous oxide” (assuming that the reaction: FeO + Fe2O3→ Fe3O4, has gone to completion) in the rock. Size distribution curves showed that each specimen contained crystals with a preferred radius and throughout the series a quantitative relation between the preferred radius and the frequency distribution about this value was observed.

Measurement of the number of crystals per unit volume showed that this gave trends with functions of the chemical composition of the rock. A discussion of the mechanism of garnet formation in terms of chemical laws of nucleation and growth is given.