H. S. Washington — Ischian Trachytes. 



377 



dine crystal has been cracked across and slightly bent, and 

 from the broken edges have grown minute needles, prolonging 

 the edges and crossing each other (fig. 1, a). 



Fig. i. 



The ground mass crystals (almost solely of orthoclase), which 

 show a decided flow-structure, have increased greatly, not 

 only in number but in size (attaining lengths of 0'3 mm ) and in 

 complexity. They are seen on examination to be elongated par- 

 allel to the a axis. !No twinned forms were seen, but almost all 

 the small prisms show more or less forked forms, some of 

 which are shown in fig. 1, b — Jc. The forms are irregular, and 

 are rather tree or twig-like, than sheaf-like, in habit, as they do 

 not show much tendency toward equal development of the two 

 ends. A number of stubby brush forms are to be observed 

 among the smallest of them (fig. 1, e, f) ; and among these 

 especially are noticed forms which are more highly devel- 

 oped and which resemble closely those to be described 

 presently. 



The basis proper is a clear glass, of a pale cafe an 'lait color, 

 showing here and there an air vesicle. Surrounding the small 

 forked crystals is a darker brown, slightly granular substance, 

 which exerts some action on polarized light, as is evidenced 

 by a very feeble aggregate double refraction. This substance 

 also occurs in isolated spots and irregular anastomosing streaks 



Am. Jour. Scl— Fourth Series, Vol. I, No 5.— May, 1896. 

 25 



