A new zeolite closely related to phillipsite and gismondine is recorded from thirty localities in the Tertiary basalts of Antrim and eastern Iceland, and the name garronite is proposed. Four chemical analyses indicate a composition of NaCa2·5A16Si10O32.13·5H2O. Garronite differs chemically from phillipsite in having a very low content of potash, and from gismondine in containing less lime, alumina, and water; optically garronite is intermediate between these two zeolites.

Gismondine is recorded from the Tertiary basalts of the North Atlantic region, from a total of some 40 localities in Antrim and Iceland. It occupies a well-marked position in the sequence of zeolite zones in the lavas, and the characteristic associates are chabazite, thomsonite, and phillipsite. Five new chemical analyses of gismondine from these localities show a negligible content of potash, and indicate plagioclase-type substitution, giving a series ranging from near Ca8Al16Si16O64.34H2O to near Na2·5Ca5·5Al13·5Si18·5O64.34H2O. Substitution of NaSi for CaAl leads to a decrease in refringence, birefringence, and density.

Feldspars, paracelsian and danburite, harmotome and phillipsite have alumino-silicate frameworks that consist of three out of the seventeen simplest ways of cross-linking chains of the feldspar type. The four-membered rings that form the feldspar type of chain are parallel and have two adjacent tetrahedra pointing one way and the other two the other way. Chains are produced by sharing oxygen atoms of oppositely-pointing tetrahedra of parallel four-membered rings. Cross-linking of chains produces frameworks containing parallel eight-membered rings. If alternate tetrahedra in the rings point the same way, a new type of chain is produced, and there are four simple ways of joining these chains together to make a framework. If all four tetrahedra in a ring point the same way, only one structure can be developed, which has been proposed already by Barrer et al. for the zeolite Na-P1 and other members of the harmotome family. If three tetrahedra in the ring point the same way, very many simple structures can be constructed, and the seven simplest of these are described. It is suggested from comparison of cell dimensions that gismondine and, perhaps, yugawaralite may belong to this series of hypotheti- cal structures. It is also suggested that some of the complexity and confusion in the harmotome group of zeolites may arise from hitherto unrecognized members of this hypothetical group. Major angular distortions from the ideal shape occur for the naturally-occurring silicates belonging to these structural groups.

The petrological and structural characteristics of four layered basic masses in Somalia are described. Sr contents of feldspars from three of the masses are similar for comparable An contents; Sr varies between 1000 and 1850 p.p.m. and tends to increase as An decreases from An68 to An48. In feldspars from the other mass Sr also increases with decreasing An content (An74 to An56), but is at a much higher general tenure (1650 to 2350 p.p.m.). It is concluded that the four masses were not all part of the same layered intrusion.

Evaluation of X-ray powder diffraction data shows that argon atoms (radius 1·9 Å) lie at the centres of the large holes in the cordierite structure. The reflection intensities for hydrous cordierite differ from those for anhydrous cordierite but to a lesser degree than those for argon-bearing cordierite. Although analysis is uncertain, it appears that the smaller water molecules (radius 1·4 Å) do not occupy either the centres of the six-membered rings or the centres of the large cavities. It is possible that they may be attached to the walls of the large cavities or occur partly as hydroxyl groups in the framework. Single-crystal X-ray analysis is necessary to resolve this problem.

Sharp exothermic peaks occurring at about 90° C during D.T.A. treatment of natural volcanic glasses are described and are shown to be due to release of external strain. This is probably triggered by loss of surface water. No systematic variation in strain could be found in glass of different geological histories, but strain is found to decrease during natural weathering of volcanic glasses. Differences in weatherability of natural glasses may be related to degree of surface strain.

The preparation of thin rock-sections using a modified high-speed surface grinding machine is described. The machine is fitted with a new type of jig designed to hold ¼ in. thick glass slides to which the rock slices are mounted for grinding. The machine operates automatically once a cut is set and the large size of the jig enables fifteen 3 in. × 1 in. sections to be produced at one grinding operation without the constant attention of an operator.

Natural cobalt hydroxide is CoO.OH, not Co(OH)2 as has recently been asserted; for this mineral the name heterogenite has priority. Copper, frequently present, may be in solid solution as (Co,Cu)(O,OH)OH, or may be as finely divided malachite; for cuprian heterogenite, the name schulzenite has priority.