Re]3orts c& Proceedings — Mineralogical Society. 559 



compounds MgO . Al^O,^ (spinel), FeO . Al.^Og (hercynite), FeO . 

 Ci'aOg (chromite), and MgO . CrgOg (here named picrochromite). 

 Sub-species and varieties of intermediate composition are divided 

 off in symmetrical areas within the square. 



L. J. Spencer: "' Fibrolite (= Sillimanite) as a Gem-stone, from 

 Burma and Ceylon." Water- worn, prismatic crystals from the 

 ruby mines in Upper Burma measure up to 1| cm. in length, and are 

 clear and transparent, with a pale sapphire-blue colour and marked 

 pleochroism. A fine, faceted gem cut from this material is shown in 

 the British Museum collection of minerals. Determinations were 

 given of the optical constants ; the birefringence shows a mde 

 range, 7-0. being seventeen times /3-«. On a somewhat similar, 

 but etched, crystal from Ceylon the axial ratios were determined. 

 Other crystals from Ceylon are pale greyish-green with a marked 

 chatovancy. 



Dr." J. W. Evans : " The Origin of the Alkali Eocks." The 

 alkali-igneous rocks form an exceptional series varying in composition 

 from acid to basic, characterized by a high percentage of alkalies, 

 especially soda, and a deficiency in alumina and the oxides of the 

 divalent elements. They appear to occur mainly in areas where 

 the earth's crust has, as the result of ancient folding or the accumula- 

 tion of granitic rocks, consolidated to a considerable depth, and where 

 the temperature gradient is abnormally low. Such are as are rarely 

 subject to new folding, but are frequently faulted, and with these 

 faults the alkali rocks appear to have a genetic relation. In such 

 areas crystallization must proceed in the sub-crystal magmas, 

 which are believed to be basic in composition under exceptional 

 pressure, with the result that minerals with low specific volumes, 

 having regard to the materials of which they are composed, will 

 preferentially crystallize out. Garnet, zoisite, fibrolite, and 

 kyanite are examples, the materials of which crystallize out under 

 less pressure with greater specific volumes. As these minerals are 

 mainly silicates of aluminium and the divalent elements, the un- 

 crystallized residue will be poor in these constituents and rich in the 

 alkalies, especially soda, which was present in the original magma 

 in greater proportion than the potash. It will also contain the 

 volatile fluxes in large amount. As a result of the faulting of the 

 crust, this residue may be pressed out, find its way upwards, and 

 give rise by further differentiation to the alkali rocks. 



A. F. Hallimond : '' Monticellite, from a Mixer Slag." The 

 crystals, which are essentially monticellite containing about 20 per 

 cent of olivine in solid solution, have the following physical 

 characters : orthorhombic, a : b : c = 04382 : 1 : 0'5779 ; forms 

 010, 110, 021; refractive indices, 1-663, 1-674, 1-680; 2V 73J° ; 

 specific gravity, 3-20. 



Dr. H. H. Thomas and A. F. HaUimond : " A Refractometer for 

 the Determination of Liquid Mixtures." A telescope and collimator 

 with Websky signal are fixed in alignment ; between them is inserted 



