OX PYROXENIC COMPONENTS IN CERTAIN VOLCANIC ROCKS. 79 



Andésite from Arequipa in Peru ; and Judd,* as occurring in the lavas 

 of Krakatoa. 



The narrow band of Augite just described may be also recognised 

 by the fact that it is traversed by fine fissures running parallel to the 

 direction of the vertical axis, and at right angles to it. Under polaris- 

 ed light Augite shows a decidedly stronger interference-colour than 

 the Rhombic Pyroxene, so that the marginal bands are in vivid con- 

 trast with the median portion. 



With regard to the microscopic characters of these crystals, the 

 longitudinal section shows traces of prismatic cleavage-fissures running 

 parallel to the longer sides. The basal section is quadrate, formed by 

 the traces of the faces, ooP^b , ocPàb , and c©P at the corners, the 

 cleavage-traces parallel to the last face being very distinct. The 

 cleavage parallel to ooPco is also occasionally observed as fine fissures. 

 The basal section sometimes shows a peculiar zonal structure resulting 

 from isomorphous layers. The central portion has, as shown in figure 

 12, a nucleus in its centre and is divided into two parts. It is distinctly 

 pleochroic, being brownish Ij 1 a axis, and greenish-brown H 1 ß axis. 

 The outer zone is of a faint brownish colour, and pleochroism is almost 

 inappreciable. The central portion, which is evidently more ferrifer- 

 ous than the outer, must have crystallized before the less ferriferous 

 portion. The crystals in general do not show any sign of alteration 

 whatever. 



Enclosures. — The most frequent enclosures are gas-pores, and glass- 

 enclosures with fixed bubbles. The symmetrical arrangement of these 

 enclosures is very characteristic. Figures 4, 5, 7, 9 illustrate the 

 typical forms of this arrangement. The most frequent form is 

 that shown in fig. 7, 9, the enclosures being arranged at both ends 

 of the crystal conforming to the faces of the pyramid i = 2?i, i. e., 



* The lavas of Krakatoa,— Geo]. Mag., Dec. Ill, vol. V, 1888. p. 1. 



