Ransome.1 



Eruptive Rocks of Point Bonita. 



«3 



Under the microscope (Fig. 5) the rock is seen to consist of a 

 finely matted groundmass of lath-shaped and acicular plagiocla.se, 

 with considerable glass, in which are scattered the porphyritic feld- 

 spars already noted. The latter sometimes possess sharp crystallo- 

 graphic boundaries, but are often rounded and corroded, or show 

 jagged step-like outlines, with rims extinguishing differently from 

 the bodyof the crystal. In general they are two much decomposed 

 to admit of accurately measuring the extinction angles of adjacent 

 lamellae, but one crystal, less decomposed than the others, gave 

 angles of about fifteen degrees on either side of the twinning plane. 



Figure 5. — Section of spheroidal basalt composing large spheroid shown in 

 PI. 7, Fig. 2. The amygdule is filled with chlorite, x 21. 



They are full of alteration products, chlorite frequently being 

 observed to replace a portion of one lamella, while the lamellae on 

 each side are not replaced, thus giving rise to rectangular green 

 inclusions. Instances were observed where a porphyritic crystal had 

 been broken in the magma and the fragments displaced, and one case 

 in which two crystals had apparently been squeezed together. Small 

 round amygdules are moderately abundant, and are generally filled 

 with beautiful aggregates of chlorite; some, however, have an outer 



