Geological Society of London. 233 



4th. The residuum of the original dolomite or dolomitic lime- 

 stone was removed, leaving the perfect and imperfect rhombohedral 

 cavities. 



A calciferous, gold-bearing quartzite from the Zululand gold-fields 

 is described and a similar origin is ascribed to it, but in this case 

 the original rock appears to have been simply a limestone, not a 

 dolomite. The gold seems to occur chiefly in the calcareous portions 

 of the rock. The author has also been tempted to suggest a similar 

 origin for the saddle-reefs of the Bendigo gold-field. In all of these 

 cases the train of reasoning is based upon the conclusions arrived at 

 in his previous paper "On the Dwindling and Disappearance of 

 Limestones." He indicates that the stratigraphical relations of the 

 Arkansas novaculites, as described in Mr. Griswold's Report, are 

 such as to warrant the assumption that limestones once occurred in 

 the position now occupied by beds of novaculite. Many collateral 

 matters are dealt with in the paper which cannot be given in 

 abstract ; among them is an attempt to classify quartzites. 



2. "Note on the Occui-rence of Perlitic Cracks in Quartz." By 

 W. W. Watts, Esq., M.A., F.G.S. 



The author of this communication describes some specimens of 

 the porphyritic pitchstone of Sandy Braes in Antrim, which are 

 deposited in the Museum of Science and Art in Dublin, and in that 

 of Practical Geology in Jermyn Street. They exhibit admirable 

 examples of perlitic structure in the brown glassy matrix, and the 

 presence of polygonal, circumferential, and radial cracks is noticed. 

 The porphyritic crystals of quartz are traversed by curved fissures 

 of retreat, not so perfect as those found in the glass, but better than 

 those usually produced by the rapid cooling of Canada balsam. The 

 fissures in the quartz are frequently prolonged into the matrix, 

 undergoing only a very slight and almost imperceptible deviation 

 in direction at the junction. But in addition to this the quartz is 

 often found to act as a centre of strain, the inner cracks of the 

 perlite being wholly in quartz, the next traversing both, and the 

 outer ones in glass only. In other examples the outer cracks of a 

 matrix perlite sometimes enter the quartz, while in others polygonal 

 cracks occur, and join up in, the quartz and give off radial cracks 

 precisely like those of the matrix. These observations lead to the 

 conclusion that the quartz and glass must have contracted at about 

 the same rate, and that the observation of perlitic structure in a 

 rock with trachytic or felsitic matrix by no means proves that the 

 rock is necessarily a devitrified glass. References are given to 

 somewhat similar observations by Fouque and Michel-Levy, and 

 by Iddings. 



III.— April 11th, 1894.— Dr. Henry Woodward, F.R.S., President, 

 in the Chair. The following communications were read : — 



1. " Mesozoic Rocks and Crystalline Schists in the Lepontine 

 Alps." By T. G. Bonney, D.Sc, LL.D., F.R.S., F.G.S., Professor of 

 Geology in University College, London, and Fellow of St. John's 

 College, Cambridge. 



