102 



University of California. 



[Vol. i. 



when radially placed, either spread apart as they approach the 

 periphery, or else the spaces become filled with what appear to be 

 still smaller microlites arranged at right angles to the principal rays. 



The radial grouping about a line instead of a point, results in 

 rather curious interference figures with crossed nicols. When the 

 longer axis of the aggregate is parallel to a principal section of one 

 of the nicols, the figure consists of two parallel black lines at right 

 angles to this axis, accompanied by two dark brushes which are 

 outside of, and perpendicular to, these lines. As the stage is rotated 

 in one direction, the brushes vanish, the bars open into hyperbolas, 

 and, when the stage has rotated 90°, new parallel lines appear in the 

 center of the figure at right angles to their former position. If the 

 rotation be in the other direction, the phenomenon is reversed, the 

 hyperbolas straighten back into bars, and the parts of the figure 

 move from without inward. 



The groundmass is wholly free from the pseudocrystallites 

 which are so conspicuous in the varioles, and contains very few 

 of the more massive bronze-brown aggregates. The varioles gener- 

 ally grade abruptly into the groundmass without any sharp micro- 

 scopic boundary; but they are sometimes separated by cracks * as 

 shown in the figure. The crack, however, does not always adhere 

 to the true surface of the variole. 



Small crystals of iddingsite, often showing sharp crystallographic 

 boundaries, occur scattered through groundmass and varioles. 

 They are generally somewhat decomposed, but the hexagonal sec- 

 tion shown in Fig. 1 1 is comparatively fresh and transparent, show- 

 ing the cleavage in a system of sharp parallel lines instead of open 

 gashes. They gelatinize readily with acids and show the optical 

 properties of iddingsite. The crystals contain numerous micro- 

 scopic inclusions, some of which are magnetite; but others are dark 

 brown in color, semi-transparent, singly refracting, and apparently 

 of octahedral habit. They are not affected by hot hydrochloric 

 acid, and are probably picotite or chromite. The presence of idding- 

 site in this spheroid is an argument in favor of its being a variation 

 of the diabase, as this mineral has not been detected in the lower 



* E. Geinitz, Ueber einige Variolite aus dem Dorathale bei Turin. Tscherm. 

 Min. u. Petr. Mitth., Bd. I, 1878, p. 136. 



