REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 



165 



Another mode of grouping is seen in fig. 31 ; the face il/of II is superposed on the 

 face P of I. This section is nearly perpendicular to the edge P/3I of I ; other forms 

 of twinning and different groupings can be seen, such as are represented in fig. 32. 



Fig. 31. — Andesite of Camiguin. 



Section of plagioclase, nearly perpendicular to PjM. 



j^ crossed nicols. 



Fio. 32. — Andesite of Oamiguin. 



Section of plagioclase, Baveno, albite, and pericline twins. 



i's crossed nicols. 



This (fig. 32) shows I, Baveno twin ; II, twin crystals of albite ; I and III, twin 

 crystals of pericline ; I, IV, Baveno twin. 



Fig. 33 shows I and II Carlsbad twin and one of albite ; the extinctions for I are 

 35° on the average (32° to 38° for a, and 32° for b). The section then approaches the 

 face a; for this individual. The individual II extinguishes at 10° for a and 6° for b ; 

 therefore the section for II approaches the face P. 



I 



prr' 



Fig. 33.— Andesite of Camiguin. 



Section of plagioclase, albite and Carlsbad twin. 



ij'g crossed nicols. 



Fig. 34. — Andesite of Camiguin. 



Section of plagioclase following M, pericline twin. 



sV crossed nicols. 



The section shown in fig. 34 is parallel to M, and shows a pericline twin. The 



