Abstract

Dr. M. H. Battey said that the “margined sacs” discussed in this paper appeared to be in many respects analogous to pillow-lavas with their differentiation into core, marginal zone and selvedge. The problem of the great differences between the bulk chemical compositions of selvedges and of the interiors of lava pillows was of long standing—at least since Slavik drew attention to it at the International Congress of 1926. Among the examples since described, a chlorite composition seemed to be the most usual. The present author found his sac margins relatively high in lime, but this was by no means always the case in selvedges, although one extraordinarily lime-rich example (20 per cent CaO) had been described from Tasmania. The speaker had himself analysed one composed almost wholly of very finely divided biotite, in which the perfect preservation of perlitic cracks showed that the original material solidified as a glass. An originally glassy condition was inferred for several of the described examples.

Metasomatism there must be, to explain these rocks ; but in view of the compositional variation in the rocks displayed, and the fact, of course, that there were examples where the marginal material represented the bulk composition of the lava as a whole, it would seem extremely doubtful whether a theory of liquid immiscibility such as that sketched in the paper could explain them, since the mechanism could presumably only apply within a definite range of composition.

If, as the author supposed, the separation of an immiscible Ca–