REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 



159 



which must be referred to in detail. Fig. 29 shows a section of plagioclase with 

 hemitropic lamellae, following the albite law. These belong to two principal individuals, 

 which mutually penetrate each other, and present, each in its turn, a larger development 

 in the different parts of the section. The two principal individual crystals, which 

 sometimes form the groundwork and sometimes the lamellae, are twinned in the following 

 manner : — 



Fig. 29.— Andesite of Ternate. Section of plagioclases, albite, pericline, and Carlsbad twin. 



I. II. . . . Albite twin. 



I. 1/ . . . Pericline do. 



II. II'. ... do. do. 



III. (I. II.) . Carlsbad do. 



Cleavage parallel to the face P is noticeable in both individuals. This is shown in 

 the figure by lines sensibly perpendicular to the albitic lamellae. Extinction takes place 

 at 33° to 34° from on the trace of M. The polysynthetic lamellae following the pericline 

 law (I'. IF), extinguishing at 27°, meet at an angle corresponding exactly to the trace of 

 PP', which is clearly indicated at the lower jtart of the figure. The third individual (III), 

 joined to the preceding group in the plane M, must be considered as forming a Carlsbad 

 twin with (I. II) ; in fact, this individual gives an asymmetric extinction at 20°. 



The augitic sections in this rock show strong pleochroism, recalling hypersthene by 

 the tints observed. We have : — 



/? 



greenish. 



reddish yellow. 



The form of the augite crystals is not that usually found in andesites, the sections 

 being terminated by an obtuse summit very like those of bronzite. This mineral is 

 sometimes twinned, and the value of its extinction never allows any doubt regarding 

 its correct description as monoclinic pyroxene. The rock we describe has the general 

 characters of an augite-andesite ; it contains, however, small hexagonal or rhombic 

 sections of olivine. The ground-mass is a base, enclosing a great number of felspathic 

 microliths, appearing like belonites, and magnetite, which also occurs as inclusions in 

 the constituent minerals. 



