DETAILED DESCRIPTION OF SLIDES. 113 



only the prismatic cleavages, but both the pinacoidal ones. A large part 

 of the augites are twinned, and many of them show polysynthetic structure. 

 In one case in another slide, from the same region, I counted thirteen lam- 

 ellae. In many cases, as is so frequent in feldspars, the lamellas do not 

 extend entirely through the crystal. An excellent instance is represented 

 in Plate III., Fig. 15. In this slide (and many similar cases have been found 

 in others from the Sutro Tunnel) there occurs a long, somewhat ill-defined 

 section of augite, showing a single cleavage parallel to the longer axis and 

 extinguishing at an angle of 38°, yet showing planes of twinning which 

 cut the direction of cleavage at an angle of 32°. At first sight this gives 

 the impression of a pinacoidal section, and a twin with a hitherto unob- 

 served face of composition. In reality it is a section at a considerable angle 

 to the principal axis, and cutting a prismatic face nearly parallel to the 

 edge P, 30 P. The second system of cleavages does not appear in this 

 instance, because it cuts the section at a very low angle. Such sections 

 must occur in all augite rocks, but attract attention here on account of the 

 prominence of the twinning.^ 



A portion of the augite is converted into uralite. This product is 

 strongly dichroitic, light greenish-yellow in color, and of course fibroixs in 

 texture. The crystallographic orientation is often the same over considerable 

 areas, and these show the angles of extinction characteristic of hornblende. 

 In some cross-sections, too, an excessively fine cleavage at an angle of 

 about 126° can be made out with high powers. The conversion into uralite 

 seems to have proceeded with little regularity, sometimes attacking the 

 augite from the outside, and sometimes along cleavages and fractures. The 

 direction of the fibers of uralite is not in general that of the augite cleavage, 

 but usually not very far from it. 



The uralite is further often converted into chlorite of a darker green 

 color and equal dichroism. The fibers of this product extinguish parallel 



• When there is reason to suppose tbat a section showing an oblique trace of a twinning plane 

 cuts one of the prism faces lying next to the clinopinacoid parallel to the edge between this face and 

 the base, the approximate position of the section can readily bo inferred ; for if the prism angle were 

 90°, the tangent of the angle at which the trace of the twinuiug plane cuts the longitudinal striations, 

 would be equal to the sine of the angle at which the section cuts the main axis of the crystal. As the 

 angle of a P is only 87J degrees, the obserN-eil and the calculated angle will be too large, but the error 

 will reach a maximum of d| degrees only in sections at right angles to the main axis. 



Sol 



