Ransome. ] 



Eruptive Rocks of Point Bonita. 



93 



ular, but not in the least jagged or intricate, and the boundary 

 between the different minerals is perfectly sharp and clear, or at most 

 shows only such appearances of transition as would be produced by 

 the overlapping of the two in the thickness of the section. Each 

 mineral retains its optical properties up to the line of separation, and 

 the contrast is fully as marked under crossed nicols. The horn- 

 blende is all compact, there being no uralite present. 



Figure 7. — Intergrowths of augite with brown and green hornblende, with 

 included crystals of iddingsite. A, augite ; BH, brown horn- 

 blende ; GH, green hornblende ; S, iddingsite ; P, plagioclase ; 

 G, glassy base. ■ 35. 



Brown mica occurs in the same slides as the hornblende, and 

 generally shows no crystal outlines, although two or three hexag- 

 onal basal sections were observed. The latter remain dark brown 

 and nearly opaque, with but slight change of color during a revolu- 

 tion of the stage. Sections from the prismatic zone show the char- 

 acteristic cleavage and strong absorption of biotite. 



Opaque iron ore occurs in the finely crystalline phases, as specks 

 and grains of irregular form, or as small, short rods, which often lie 

 across each other like a pile of railroad ties, or the arms on a tele- 

 graph pole, and in the more coarsely crystalline specimens as irreg- 

 ular aggregates of some size, and in extremely jagged plates, like 

 those shown in Fig. 8, exhibiting a system of angles of 6o° or 

 I20°. It is also abundant in the crystals of iddingsite, generally in 

 the form of little rectangular quadrilaterals. It is either quite fresh, 

 and unaccompanied by alteration products, or else is surrounded by 

 gray clouds of leucoxene. Several examples of a shelly structure 



