H. S. Washington — Ischian Trachytes. 



379 



which seem to be detached. Closer examination shows that 

 most of these (if not all) are in reality of one substance with 

 the main group, the connection being either masked by tri- 

 chitic matter, or having been destroyed by the position of the 

 plane of the section. 



Fig. 2. 



All these needles diverge at generally not large angles (up to 

 about 30°) from the nucleus or from each other; though here 

 and there some are seen which form angles up to 90° with the 

 main prism. Though usually straight, or nearly so, many show 

 marked curvature, the concavity being always outward. In 

 some cases needles along the side form curves which are quite 

 hyperbolic in character (tig, 2, a, e,f). 



Between the needles lies a colorless, or very faintly brown, 

 isotropic substance, which is very thickly sprinkled with minute 

 globulites and curved black trichites. These bring out the 

 structure very plainly, the colorless needles standing out bright 

 against the dark background. 



Examination with the mica plate shows that in both the 

 nuclear prisms and the divergent needles the axis of greatest 

 elasticity o, lies parallel to the length, and that hence the crys- 

 tals are elongated in the direction of the a axis — the usual 

 habit. 



