go 



University of California. 



[Vol. i. 



red color of certain augites is always due to the presence of titanium, 

 which he regards as replacing iron in the sesquioxide. 



Augite of two different colors is sometimes intererown. as in 

 Fig. 8, where augite of a pale pink tinge is surrounded by a greenish 

 white variety, the two being optically continuous, and separated by 

 a fine, sharp, irregular line. The intergrowths with hornblende will 

 be described under the latter mineral. 



Iddingsite (see Figure 10, page 97) is present in many of the 

 slides of the diabase, in rounded, idiomorphic crystals of various 

 sizes, up to about 1 mm. in length, whose outlines are strongly 

 suggestive of olivine. The color by transmitted light varies from 

 light greenish yellow to a dark, dingy green. The mineral was 

 one of the earliest separations from the magma, and is often entirely 

 surrounded by augite, or hornblende, and sometimes partly so by 

 brown mica. It includes abundant grains of opaque iron ores, and 

 frequently dark brown microscopic crystals of chromite or picotite. 

 In the majority of cases the original crystallographic outline has 

 been lost, presumably by magmatic corrosion, but such sections as 

 show definite bounding planes fall generally into two classes, the 

 one showing hexagonal outlines with nearly equally developed 

 sides, accompanied by a very distinct monotomous cleavage, the 

 other also showing six-sided outlines, but elongated in one direc- 

 tion, so as to form short, stout, prismatic forms, with terminal angles 

 varying from 77 to 90 , usually nearer the latter. 



The cleavage is shown by a system of parallel open cracks or 

 gashes, about equally spaced, which subtend two opposite, and 

 generally the two largest, angles of the hexagonal sections, and are 

 perpendicular to two opposite sides. Measurements of the crystal- 

 lographic angles subtended by the cleavage cracks gave readings 

 ranging from 113 to 142 . These sections are pleochroic, being 

 dark yellowish green parallel to the cleavage, and light greenish 

 yellow at right angles to this position. Under crossed nicols the 

 undecomposed portions show brilliant mottled polarization colors, 

 crimson and green predominating, and the double refraction is 

 therefore strong. The mean index of refraction is rather low. 



The distinctly terminated prismatic sections are but slightly 

 pleochroic, and show no cleavage. The interference colors are, 



