140 Reports and Proceedings — Mineralogical Society. 



those at right angles to them to sin' |-, so that the former will bear to 

 the latter the ratio cot^ |-. If 6 be less than 35°, this will be greater 

 than 10, and tlie light from the directions at right angles may be 

 neglected both in respect of its direct effects on the Becke phenomena 

 and its indirect action in producing interference. — (4) Note on the 

 spontaneous crystallization of solutions as spherulites ; by Mr. J. 

 Chevalier. Experiments on solutions of potash - alum, sodium, 

 ammonium, and lithium sulphates, etc., made at the suggestion of 

 Professor Miers in the Oxford Mineralogical Laboratory, show that 

 spherulites and sphero-crystals are characteristic of the spontaneous 

 crystallization of many solutions in thin drops. When other crystals 

 grow first, it is probably because they have been introduced, the drop 

 in that case appearing to be metastable. The spherulites mark the 

 passage of the solution to the labile state. — (5) On a method for 

 studying the optical properties of crystals; by the late Dr. H. C. 

 Sorby. The author gives complete details of his work on the 

 determination of refractive indices in thin plates, of which preliminary 

 accounts have been published in the first two volumes of the 

 Mineralogical Magazine. The method he describes in the case of 

 doubly refractive minerals is identical in principle (though devised 

 quite independently) with that given bj^ the Due de Chaulnes for 

 singly refractive substances, but is worked out in far greater detail. — 

 (6) Some additional localities for idocrase in Cornwall ; by Messrs. Gr. 

 Barrow and H. H. Thomas. During the mapping of the metamorphic 

 area round the Bodmin Moor granite further occurrences of idocrase 

 have been found in the altered limestones. Well-shaped crystals of 

 the mineral, up to 6 mm. in length, are fairly common in drusy 

 cavities. They are perfectly uniaxial, but show in thin sections 

 considerable variation in the double refraction, especially in the outer 

 layers of the crystals. The idocrase is associated with pale -pink to 

 pinkish-brown garnet (often in regular intergrowth with the idocrase), 

 pale- green diopside, and epidote approximating to clinozoisite in its 

 low extinction and birefringence. — (7) Detrital andalusite in Tertiary 

 and post-Tertiarj' sands; by Mr. H. H. Thomas. Occurrences of 

 detrital andalusite are described in sands from vaxious localities in 

 West Wales. In no sedimentary rock of greater antiquity than the 

 Pliocene has detrital andalusite been found. In the sands of West 

 Wales the mineral occurs as slightly elongated, somewhat angular 

 grains, often showing very intense pleochroism from blood-red to pale 

 greenish blue. It is associated in these sands with pink garnet, 

 greenish-brown augite, cyanite, zircon, riitile, tabular anatase, stauro- 

 lite, brown and more rarely blue tourmaline, green hornblende, bright- 

 green epidote, cordierite, iron-ores, and in some cases glaucophane. — 

 (8) The energy of twin-crystals ; by Mr. H. Hilton. The author 

 determines in a simple case the conditions according to which 

 a twin-crystal may be a more stable form, or in other words may 

 have less surface energy, than a simple crystal of the same volume. 

 — Mr. Hutchinson exliibited a new protractor devised by Dr. V, 

 Goldschmidt. 



