1885.] Microscopic Characters of Devitrified Glasses. 423 



by artificial means. The first case considered is that of a piece of 

 thick plate glass which has been totally devitrified nnder conditions 

 described in detail in the paper. In the roughly broken specimen 

 the fractured surface sufficiently reveals the fact that crystallisation has 

 taken place throughout the mass, while other specimens subsequently 

 described by the authors demonstrate beyond question that the crystal- 

 lisation has started at the surfaces and has travelled inwards. In 

 this particular specimen the crystallisation has advanced from the 

 different surfaces until the various sets of crystals have arrested one 

 another in very definite planes, whose traces are clearly perceptible on 

 a fractured surface. A figure showing this has recently been pub- 

 lished in Prof. Bonney's Presidential Address to the Geological Society, 

 the drawing having been made from a chip taken from part of the 

 same plate of glass now in the Musenm of Practical Geology. An 

 examination of a section cnt in one direction from the specimen now 

 described, showed little more than has already been stated, except 

 that the crystals were seen to occur in bundles of divergent prisms, 

 the centres of divergence being at or near the surfaces of the plate 

 of glass. An inspection of a ground surface of the plate showed a 

 curious reticulation, in some places very well marked, and it therefore 

 seemed desirable to examine another section taken parallel to this 

 surface, and at right angles to the former section. This was done, and 

 it then became apparent that a third section mnst be cut, so as to 

 traverse two directions of crystallisation disposed at right angles to 

 each other before any sound interpretation of the phenomena could be 

 arrived at. This third section traversed one of the triangnlar areas, 

 or rather wedges formed by arrest, on the shorter side of the plate, at 

 a little distance from the thin end of the wedge, and, beyond this area, 

 it passed parallel oo the adjacent directions of crystallisation. In this 

 section the bnndles of crystals lying normal to the plane of section 

 were seen exactly to resemble those visible in the second section taken 

 parallel to one of the broad surfaces of the plate, while the others 

 were identical with those already seen in the first section. It then 

 became clear that a system of prismatic jointing had been developed, 

 presumably due to strain around separate centres of crystallisation 

 from which the divergent fasciculi emanated, while from the first and 

 third sections it was evident that this jointing extended from the 

 surface inwards, until the development of one set of crystalline bundles 

 was arrested by another set. In sections taken parallel to these 

 fasciculi there is a curious wavy brindling, which is apparently dne to 

 structure analogous to that seen in " cone in cone " ironstone and some 

 other minerals, the marked character of the phenomena when viewed 

 by transmitted light being probably due to want of coincidence in tin- 

 direction of the minute prisms or fibres composing the overlapping 

 divergent groups. In order to ascertain more fully the conditions 



