MINERALOGY. il\H 



iiom Coni\v;ill, aud jittcnipted to show thai the iiiea«ureraeiits can 

 be explained only by the iissun)i)tiou of the presence; of "vicinal" 

 planes with very complex indices, or else of a monoclinic form with an 

 obliquity of 36'. The correctness of su(;h conclusions may f.iirly be 

 questioned. The phlogopite of Templeton, Canada, which is black to 

 brown in color and shows the well known asterism, has been studied 

 by Lacroix. His results go to prove that the inclusions to which the 

 asterism has been long recognized as due are in this case minute acicu- 

 lar crystals of rutile, which cross each other at angles of 90°, lliO°, and 

 150°. Doelter has added to our knowledge of the relation between the 

 optical properties and chemical composition of the various kinds of py- 

 roxene, a subject the interest of which has already been recognized. 



The optical behavior and micro-structure of corundum forms the sub- 

 ject of an interesting paper by Lasaulx, which goes far to settle the 

 disputed question as to its true crystalline form. He concludes that 

 corundum is truly optically uniaxial and hence is to be referred to the 

 rhombohedral, not the monoclinic, system, and that the anomalous op- 

 tical characters which have suggested the latter conclusion are con- 

 nected with disturbances due to the growth and structure of the crystals. 

 The optical irregularities are further associated with structure planes 

 parallel to the rhombohedron,the base and the second hexagonal i)rism : 

 the lamellar structure in these directions conditions the cleavage or 

 " i)arting" often observed parallel to one or more of these forms. This 

 parting, he concludes, is not caused by twinning lamellse, as has been 

 assumed, but the latter determined by the presence of the structure 

 planes, both appearing simultaneously. The optical irregularities are 

 caused in part by tension in certain of the crystal, in part as a con- 

 sequence of the interposed twin-lamellae. 



Rinne has made a study of the rare silicate milarite from an optical 

 point of view, and reached the conclusion that it is to be taken as belong- 

 ing in fact to the hexagonal system, although secondary causes have 

 given it the optical structure which has led to its being classed as an 

 orthorhombic species with hexagonal pseudo-symmetry. An analogous 

 conclusion is reached by the same author in regard to apoph.yllite. 

 Stadtlander has contributed the results of an optical study of sections 

 ofapophyllite crystals from Stemi)el, near Marburg, extendingour knowl- 

 edge of the optical anomalies and showing that, as has been proved be- 

 fore, they must be referred to secondary causes. 



The often-discussed subject of the true form of leucite crystals (see 

 report for 1884) has received an interesting contribution from Kosen- 

 busch. After reviewing the results reached by others, especially by Klein, 

 he goes on to detail the results of some experiments by himself on the 

 effect of a high temperature upon the twin structure of the crystals. A 

 section of an implanted crystal of leucite from Vesuvius, showing distinct 

 twinning lamellae, was held in the platinum-pointed })incers before a 

 microscope in such a position that a strong reflected light brought out 



