344 H. D. CAMPBELL AND W. G. BROWN — COMPOSITION OF TRAP. 



Two analyses of fresh material from the " Twins " separated by the Klein 

 solution between the densities 3.105 and 3.29 gave the following results : 



Analyses of Diallagic Augite. 



I. II. Mean. Ratio. 



SiOg 49.01 49.66 49.33 ^822 .822 



AljOg 8.85 9.44 9.15 .089 \ ^^i 



Fe^Og none .54 .27 .002/-"^^ 



FeO 9.05 9.05 9.05 .126] 



MnO trace trace ! -oq 



CaO 16.94 15.89 16.36 .292 l*'®'^ 



MgO 14.51 14.66 14.58 .365 J 



K2O .19 .19 .19 .002) ^,^ 



NagO .55 .55 .55 .008 j "^^ 



Ignition .25 -25 .25 



99.35 100.23 99.73 



KO:R203:Si02 = 17:2:18. 



Characters of the Hypersthene. — Hypersthene is the most conspicuous 

 mineral in the typical specimens of this rock. The grains, when isolated, 

 are of a deep honey-yellow color. They are usually larger than those of the 

 augite, and have a better defined outline. Under the microscope they are 

 strongly pleochroic. Rays vibrating parallel to the brachydiagonal are 

 brownish-red; those vibrating parallel to the macrodiagonal are yellow; 

 those vibrating parallel to the vertical direction are green. 



In thin sections this pleochroism is distinct but not so marked. When 

 the sections are very thin it is quite feeble. The crystalline form is fairly 

 well defined, but the edges are usually irregular, owing to penetration by the 

 smaller crystals of feldspar. When cut parallel to the prismatic zone the 

 crystals are two or three times longer in the direction of the vertical axis 

 than in the other direction. The length rarely exceeds 4 mm., and is more 

 frequently only about 2 mm. Sections perpendicular to the vertical axis 

 show the prisms truncating the edges between the much more strongly devel- 

 oped pinacoids. 



Cleavage parallel to the prism of about 92°, as well as cleavages parallel 

 to the brachypinacoids and macropinacoids, can be seen in cross-sections of 

 these crystals. Imperfect cleavage perpendicular to the vertical axis and 

 others parallel to the terminal faces are seen in longitudinal sections. 



The interference colors are low compared with those of augite. 



The direction of extinction is parallel to the pinacoidal cleavages and 

 diagonal to the prismatic. Hence all sections will extinguish parallel to the 

 longitudinal direction of the crystals. 



Sections which show yellow and green pleochroism give an acute bisectrix 

 in converging light. Sections showing prismatic cleavage at right angles 



