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Two parallel belts of late Mesozoic ophiolitic rocks occur along the outer arc ridge and inner volcanic line of the Indonesian arc system. The northernmost occurrence along the outer arc ridge is in the Andaman group of islands where ophiolitic rocks occur as dismembered slices emplaced over Eocene to Oligocene turbidites.Volcanic members of the ophiolitic suite occur in two thrust slices, one consisting only of pillow basalts while the other is made up of basalts, basaltic andesites and acid differentiates. Certain chemical characters of the basaltic components are comparable to MAR basalts at 45" N. On the basis of comparable REE patterns and the presence of appreciable volumes of acid differentiates, they are further suggested to represent marginal basin crust. Variable (La/Ce), and (La/Sm), ratios in these basalts are interpreted in terms of a heterogeneous source.The basaltic melts have undergone extensive fractional crystallization of clinopyroxene, olivine and plagioclase to produce basaltic andesite and acid volcanic rocks. Between the parent and the differentiates, there is no appreciable change in (Ce/Yb), ratio or HREE abundance level, suggesting low pressure fractionation in a shallow magma chamber, possibly beneath a slow spreading centre.

Solute electrolyte and adsorbed emulsifier can be removed by ion exchange from polystyrene latices prepared with persulphate initiator. Rigorous purification of the ion-exchange resins is necessary to avoid contamination by leached polyelectrolytes. These ion-exchanged latices are stabilised by strong acid surface groupssulphate end-groups of the polymer molecules. The number of surface groups as determined by conductometric titration of the latex, plus the number buried within the particle as determined by conductometric titration of the polymer in solution (or by X-ray fluorescence), is less than the expected value of two per polymer molecule. The other end-groups are shown to be hydroxyl groups by oxidation to the carboxyl form followed by ion exchange and titration. The total number of end-groups thus determined is close to two per molecule. As the pH of the polymerisation medium is decreased, the proportion of hydroxyl end-groups increases and that of the sulphate end-groups decreases. No carboxyl groups are detected in ion-exchanged latices prepared in a nitrogen atmosphere, independent of the nature of the emulsifier. The dye partition method gives end-group concentrations that are far lower than those determined by titration or X-ray fluorescence, probably because the formation of the dye end-group ion-pair is not complete. Finally, for latices prepared at relatively low emulsifier concentrations, the contribution of the sulphate end-groups to the surface charge density is significant and cannot be overlooked, as has been done in the past.

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