LONDON Mineralogical Society, March 23.—L. J. SPENCER: Biographical notices of mineralogists recently de ceased (fifth series). MAX H. HEY: A possible source of error in the determination of symmetry from optical extinction-angles. In certain cases a small departure of a cut plate from the intended section direction may lead to comparatively large errors in the extinction angle and hence to an incorrect determin ation of symmetry. This is well illustrated by mesolite. MAX H. HEY: Studies on the zeolites. (5) Mesolite. New analyses and X-ray studies of mesolite indicate that the correct formula is Na4Ca6Al16Si24O8022H2O. There is often a slight replacement of sodium by potassium, and generally an appreciable replace ment of calcium by two atoms of sodium. The axial ratio has been determined by goniometric and X-ray methods. Refractive index, birefringence and optical axial angle measurements have been made. The vapour pressure has been studied by the isohydric method previously described. A number of base exchange products have been prepared and the potassium and lithium derivatives shown to be identical with those obtained from natrolite. This provides the first conclusive proof that mesolite and natrolite are, as has been commonly assumed, iso-structural. X-ray photographs of mesolite are very similar to those of natrolite, but show distinct differences. The space-group is C. A. E. MOURANT: The dehydration of thomsonite. A study of the dehydration of thomsonite by the isobaric method. The results, obtained some years ago, are supple mentary to those obtained by Hey by the isohydric method and differ from them in some respects. Dehydrated thomsonite does not absorb air. The lattice-shrinkage reaction has been further investigated.—F. J. TURNER: Note on the occurrence of piedmon-tite in quartz-muscovite-schist from the Shotover valley, western Otago, New Zealand. A description of piedmontite in schist occurring as boulders in the Shotover River. The mineral has not previously been recorded from New Zealand.