90 Messrs. D. Herman arid F. Rutley. 



Fig. 1. 



In order to ascertain more precisely the nature of this structure, 

 three sections were cut : — 



A, at right angles to the parallel faces of the plate. 



B, parallel to one of the parallel faces of the plate, and including 



between the planes of section little more than the greenish 

 superficial layer with the white reticulations.* 



C, parallel to one of the short sides, passing through one of the tri- 



angular areas near one >of the angles, so that the section 

 includes both sets of bands, cutting one set transversely and 

 lying parallel to the other. These sections exhibit the general 

 structure admirably when held obliquely in different directions 

 between the eye and the light. 



The following is a description of their microscopic characters : — 

 Specimen No. 115, Section A. When viewed by ordinary trans- 

 mitted light under a low power, the banding already described is 

 indicated by transparent or very translucent belts, alternating with 

 bands of very feeble translucency. Both kinds of belts are traversed 

 by fine lines, indicating a fibrous crystalline -structure, which, com- 

 monly shows a radiate arrangiement, the divergent groups of fibres 

 emanating from centres situated on or about the margin or edges of 

 the section."|" Each radiating group has what may be termed its -own 

 allotment, bounded by well-defined straight lines. The boundary of 

 one side of an allotment sometimes consists of a single straight line, 

 at others of two or more straight lines, meeting in very obtuse angles. 

 The boundary lines are not curved. When the section is rotated 

 between crossed Nicols, these allotments form a well-marked and im- 

 portant feature. The divisional lines in this section may be separated 

 into two groups. The first group consists of five lines, viz., a 

 median line, running parallel to the two parallel faces of the plate- 

 glass, and four lines which form a bifurcation at either end of this 

 median line, and enclose the terminal triangular areas. The second 

 group of lines consists of those boundaries of the crystalline allot- 



* This layer is not the original surface, of an inch having since been ground off. 

 + It must, however, be noted that the margin of the section lies about of an 

 inch from the original surface, the of an inch hav.ng been removed by grinding. 



