Reports & Proceedings — Minerdlogical Society. 139 



They fall into two main divisions, distinguished by the absence 

 or by the presence of porphyritic felspars. Those of the non- 

 porphyritic class are the most prevalent, arid usually form the central 

 portion of sills or inclined sheets. The marginal portion of these 

 intrusions is crystalline or stony. The petrological characters of 

 these pitchstones and their more crystalline margins are such that 

 they seem to warrant the grouping of the rocks under a new type- 

 name, and the name leidleite has been chosen. The porphyritic 

 pitchstones occur as flat or gently inclined sheets ; they also are 

 associated with a more crystalline phase, and have been grouped 

 under the type-name inninmorite. 



The relation of the stony margins of the pitchstone intrusions to 

 their glassy centres is usually seen clearly. A typical leidleite may 

 have 5 feet of pitchstone in the centre, with margins of stony matter 

 3 feet thick on each side. Occasionally the central glassy portion 

 may be split up by stony partings. 



A feature that occurs very frequently in these rocks is what has 

 been termed 'sheath and core' structure. In this case the stony base 

 and top of an intrusion send off narrow sheets of stony character 

 which traverse the glassy portion in a branching and sinuous manner. 

 The glassy nature of the cores is clearly not due to a greater rapidity 

 of cooling; but, with the object of ascertaining the reason for the 

 devitrification, a chemical investigation of both the glassy and stony 

 portions was undertaken. 



It has been found that there is a much greater percentage of water 

 given off from the rock at 105° C. in the case of the glassy variety, 

 and the authors suggest that the escape of this excess of water, soon 

 after the consolidation of the rock, has resulted in the devitrification 

 of the sheaths and margins. 



II. MlNEEALOGICAL SoCIETr. 



January 26, 1915.— Dr. A. E. H. Tutton, F.R.S., President, in the 



Chair. 

 S. Kozu : The Dispersion of Felspar. By the most refined methods 

 the dispersions in the three principal directions were determined for 

 various members of the felspar group. — F. P. Mennell : Note on the 

 Colour of some Alluvial Diamonds and of Pyrrhotite. The colour, 

 usually green, of the diamonds in the gravels of Somabula, Rhodesia, 

 is superficial, and probably due to infiltration, presumably of iron 

 salts, while the stones were lying where they now occur. Pyrrhotite 

 is tin-white in colour when fresh. The cause of its rapid alteration 

 was discussed.— Professor G. Cesaro : Crystals of Calomel from 

 Spain. The crystals, which were pale - yellowish in colour, 

 imperfectly transparent, from 1 to 3 mm. in size, and displaying the 

 forms 100, 111, 311, were most irregularly developed. — Professor G. 

 Cesaro : General Formula for the Birefringence of a Crystal-plate in 

 terms of the Angles which its Normal makes with the Principal 

 Optical Axes. The approximate formula is obtained by supposing 

 the mean index of refraction to become infinite, while the differences 

 between it and the greatest and least indices remain constant. — 



