92 



Messrs. D. Herman and F. Rulley. 



the thick plate-glass.* If so it is possible that the strain consequent 

 on crystallisation may have produced the prismatic fission. Fig. 1, 

 Plate 3, might then be taken to represent portion of the surface of 

 the slab at the commencement of devitrification, the dots indicating 

 primary centres of crystallisation, while fig. 2 on the same plate 

 would represent the development of prismatic structure by the forma- 

 tion of cracks between and around these centres of crystallisation. 

 Fig. 3, Plate 3, shows one side (the lower one) of the block, fig. 2 the 

 arrow denoting the direction in which the crystallisation advances. 

 Apart from any hypothesis concerning the possible relation of the 

 prismatic structure to the crystallisation, which may or may not be 

 true, since it is possible that the prismatic structure was developed 

 first, it is evident from the inspection of such a diagram that we may 

 have a section giving prisms of very different widths, the width in 

 section not necessarily representing the actual width of the prism, 

 while in such a case the centre and general axis of the crystalline 

 bundle may appear to be thrown on one side of the prism. 



Specimen 115, Section C. This section truncates one of the ter- 

 minal triangular wedges, of which mention has already been made, so 

 that here we know for a certainty that we are looking on the cross 

 section of the crystalline fasciculi belonging to the triangular area, 

 and here we meet with precisely the same phenomena as those 

 described and figured for Section B. On either hand the adjacent 

 crystalline bundles emanating from the upper and under surfaces of 

 the thick plate are seen lying in the plane of section, i.e., we are 

 looking at longitudinal sections of those bundles. In these we 

 again see the cat-like brindlings. On the broken and partly ground 

 away edges of this part of the section, a power over 500 linear shows 

 that the crystalline bundles are made up of small fibres or microliths, 

 closely packed side by side. The section is in all parts traversed by 

 long, fusiform, or acicular brownish microliths, which lie with their 

 longest axes in various directions, but usually across the general 

 directions of crystallisation. 



The brindled appearance in the crystalline bundles of these sections 

 suggests at first sight the idea of pauses in the crystallisation, but 

 when we find that by ordinary illumination the light is very faintly 

 transmitted along these lines, some further explanation seems need- 

 ful, and it seems probable that in these diverging crystallisations there 

 is a kind of cone-in-cone or divergent composite structure, such as in 

 that met with in the kidney-ore variety of hematite, or in clay -iron- 

 stone, the apices of the cones giving rise to a turbidity and being 

 ranged so as to form successive arcs of approximately concentric 

 circles, as indicated in fig. 6, Plate 3. From the evidence afforded by 



* The surface of this specimen is y 1 ^ of an inch from the original surface, which 

 has been removed by grinding. 



