Summary. Mechanisms forming silicic segregations from basaltic magmas are considered of primary importance when dealing with magmatic problems. However, the processes which give rise to silicic segregations from basaltic magmas are so far obscure. Fortunately, the discovery of spheroidal felsic masses in some basic rocks of Western Sicily throws light on this subject. To clarify the relationships between felsic and basic fractions particular attention has been paid to the interactions which occurred at their contact. Textural evidence indicates that the accretion mechanism of the Sicilian felsic segregations tends to obliterate the silicate liquid immiscibility effects and suggests that the formation of silicic segregations is a consequence of liquid unmixing phenomena.

Summary. Stony clays, comprising weathered bryzoan-rich chert and silica clay, are found adjacent to Carboniferous Limestone on the west and a weathered dolomitized zone on the east near Birr, County Offaly, Ireland. A gravity survey together with a stratigraphy obtained from boreholes and other test borings suggests a depth of about 40 m and a surface width or diameter of approximately 60 m for the deposit. Indirect evidence points to a possible Tertiary age. The interpreted origin of the stony clays is by silification of Carboniferous Limestone and preservation by solution subsidence along a possible fault zone.

Summary. Upper Riphean/Vendian basalt lavas within the Hedmark Group of the Osen-Røa Nappe Complex, the lowest tectonostratigraphic unit within the Scandinavian Caledonides, are typical continental tholeiites. Differences in their REE patterns may reflect source heterogeneities, while the general lack of correlation between incompatible trace element ratios may be due to contamination of the basalts by continental crust. Their association with fluvial deposits in NNW-trending graben, and some geochemical disparities with time-equivalent basalt dykes of MORB-like character in tectonically higher units, may indicate their formation in an aulacogen related to the central rift zone of the lapetus Ocean.

Summary. A distinctive level of bedded, internally laminated, deep-sea cherts in the area west of Città di Castello, central Apennins, has been studied. These cherts form part of the Lower Miocene Monte Santa Maria Tiberina Formation, and belong to the Tuscan sequence, consisting of pelagic and turbidic sediments. The sediment is enriched in diatoms and Radiolaria. The diatoms provided silica as well as barium, now present as submicroscopical baryte concretions. Synsedimentary volcanism could have provided silica as well, through calcitization of volcanic fragments within the sediment. Since the laminations display early diagenetic microdeformations,. it can be concluded that the cherts themselves are an early diagenetic feature.

Summary. Microsolena thurmanni Koby and M. foliosa Roniewicz (Scleractinia) are important rock-formers in the Upper Oxfordian Stage of Upware and, to a lesser extent, Yorkshire.

Summary. The available data on radiometric dating of Riphean and Vendian deposits in the European part of the U.S.S.R. enabled us to establish the age of geochronological boundaries. The Riphean lower boundary was adopted as 1650 ± 50 Ma, the Vendian lower boundary as 650 ± 10 Ma, and the Cambrian lower boundary as 590 ± 10 Ma, making the total length of the Riphean about 1000 Ma and that of the Vendian about 60 Ma. This paper suggests a draft of the geochronological scale of the Late Precambrian.