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The chemistry of smectites from some bentonite deposits derived from intermediate rocks has been examined by electron microprobe methods. A large variation in chemical composition within very short distances, principally controlled by a well-defined negative relationship between Si and A1, and between A1VI and Fe 3+ and A1VI and Mg has been observed. On the other hand, Mg does not vary systematically with either Si or Fe3+. In several bentonites beidellite coexists with montmorillonite and there is a compositional transition between the two smectite minerals, implying the existence of a possible solid-solution series. This transition occurs only when Cheto-type montmorillonites are present, being absent for Wyoming-type montmorillonites. No compositional transition between Wyoming-and Cheto-type montmorillonite was observed. It is believed that the compositional variations reflect initial chemical gradients originated during the devitrification of the volcanic glass, due to the migration of chemical components.

SUMMARY. Electron-microprobe analyses of cumulus olivine, chromite, pyroxene, and plagioclase from layered peridotites and allivalites of the Eastern and Western Layered Series of Rhum demonstrate the presence of cryptic variation. Olivine varies from Foas_78 within individual units, and there are corresponding changes in the Mg/(Mg+Fe 2 § ratios in the pyroxenes and chromites. Plagioclase changes are not so dramatic, but the An-content broadly follows the Mg/(Mg + Fe 2 +) ratio in the other minerals. The most Fe-(and Na-) rich phases do not occur at the top of lithological units, but some way below. The composition trend above them is reversed. The data are interpreted as the result of periodic infilling of a magma chamber, the new magma mixing with the remains of the previous pulse. Each pulse was followed by a period when fractional crystallization produced the layered rocks. New data on Ni in the olivines suggests that the ratio of the volume of initial magma to volume of layered rocks was about four to one, the initial magma being allied to the high-calcium low-alkali tholeiitic basalts of Skye.THE Isle of Rhum has long been noted for its .layered ultrabasic rocks, which form a roughly cylindrical mass (7 km in diameter) emplaced within Lewisian and Torridonian country rock during Tertiary igneous activity. These ultrabasic rocks consist of a sub-horizontal alternation of olivine-rich (peridotite) and plagioclase-rich (allivalite) layers, which were originally thought to represent a succession of sill-like injections of peridotite and allivalite magma (Harker, I9o8), but are now interpreted as a cumulate sequence dominated by a large-scale alternation of olivine (-chrome-spinel) cumulates and olivine-plagioclase (-clinopyroxene) cumulates (Wager and Brown, I968). Each peridotite layer and the overlying allivalite layer are thought to be the complementary products of a single major episode of crystal accumulation, and are referred to as cyclic units (or units, in brief). Two distinct stratigraphic successions have been identified. In eastern Rhum, fifteen units are represented in a 75o m sequence forming the twin peaks of Hallival and Askival (Brown, i956 ) . These are termed the Eastern Layered Series (ELS). Individual units (numbered Copyright the Mineralogical SocietyI-I 5 from the lowest upwards) vary between 15 m and I5o m in thickness, and the peridotite member is generally considerably thicker than the allivalite. In south-west Rhum the succession reaches a thickness of approximately 14oo m, but comprises fewer cyclic units (Wadsworth, t96I). Three complete units (B, C, D) have been recognized, and these are underlain by cumulates believed to represent the top of another unit (A). Together they form the Western Layered Series (WLS), with the individual units between 3oo m and 5oo m in thickness. Peridotite is even more dominant than in eastern Rhum. The relationship between the ELS and WLS is not at all clear. There is a region of considerable structural complexity between the two, and this has not ye...

Thirty analyses of each of nine well-characterized silicate glasses and minerals have been made, using a Link Systems Model 290-2KX energy-dispersive spectrometer, fitted to a Cambridge Geoscan. Statistical analysis of the analytical data shows that the accuracy and precision of energy-dispersive analyses are comparable to wavelengthdispersive analyses, provided that more than about 1 wt % of each element of interest is present. The detection limits for different elementsvary according to the nature of the sample, the element concerned and the counting time. For 100 liveseconds counting times the detection limits are in the range from 0.05-0.26 wt % of element. program for processing electron microprobe analytical data. EMPADR VII.