OX COllDIElilTE AS CONTACT MINERAL. 321 



lines at right angles to it are peculiar lines of structure, which are 

 represented by fine clefts or by the parallel arrangement of foreign 

 matters, chiefly carbonaceous particles. On close examination, we 

 can often observe the indentation at the corners of the transverse 

 section, formed by the intersection of the macropinacoidal faces of the 

 neighboring individuals. Figure 12 shows an example in which the 

 twin-individuals are widely separated from each other by the de- 

 velopment of the macropinacoidal faces. 



The longitudinal section always presents a rectangular outline. 

 Aery often both extremities are a little narrower than the middle 

 portion, presenting a fusiform appearance. It is always intersected by 

 two diagonal lines, which are formed by the accumulation of coaly 

 particles, crossing at an angle of nearly 20°. The section is thus divided 

 into an inner and the outer; the former being included by the acute, and 

 the latter by the obtuse angle of the diagonal lines. Such a structure 

 is analogous to that observed by Hussak* in the Asama Cordierite ; 

 the diagonal lines of boundary being, in this case, made up of the 

 chain of enclosures, which consist of irregular Ausnte and Magne- 

 tite grains and glass particles. 



The structure of the inner and the outer portions are somewhat 

 different. The inner portion is always fibrous in structure ; the direc- 

 tion of fibres being arranged parallel to the longer side (Fig. 4, 5.) This 

 must correspond to the central core of the transverse section which 

 we have already described, and the fibrous appearance is no doubt due 

 to the intermingling of the three systems of individuals in the twin- 

 ning position. Examined under polarized light, the different fibres 

 are different in colour, so that a beautiful display of the 

 variegated bands is observed ; this being due to the fact 



* 1. c. p. 343 



