40 KOTO : XOTES ox THE GEOLOGY 



the general microscopic aspect being a crystalline Andésite -like. 

 They are all extremely ricli in augite, and the structure is (/rami - 

 Utic, Tlie felspars are of two generations {PL II, Fig. 2 and 3), 

 and the rock is consequently porphyritic, owing to the presence 

 of a few large cr3^stals of plagioclase, though this structure could 

 not be easily recognised as such in the ])resent group. 



The p/ienocrystic plagioclase is narrow-tabular with a few twin- 

 ning lamellœ (see Iig. 3), and is remarkable in its being traversed 

 through by sets of cracks which run approximatel}^ parallel with 

 each other. In one instance, only one lamella, out of many twin- 

 ned parts after the pericline law, is provided with closely set fissures. 

 This anomalous feature can be seen in all the specimens of the 

 present type, but not common in others, and the same peculiarity 

 recurs also in augite whose granular aspect is due in great measure (o 

 the same cause. I cannot offer at present a satisfactory explanation 

 to account for this phenomenon ; but, as Judd says, it might in 

 part have been cansed by a slow but constant movement of a 

 crystallizing magma, and also chilled suddenly, perhaps by the 

 access of water at the final stage of consolidation. I may here 

 adduce in support of my ground a fact of the special distribution 

 of the Hypersthene-basalt which, so far as I am aquainted with, 

 occurs only in the outlying islets, excepting the locality Sei-kei, 

 on the north coast of Hoko, which also lies not very far from the 

 present sea-shore. 



Hyperlhene occurs exclusively, though insignificant in quan- 

 tity, in the form of phenocryst {Fl. II, Figs. 2 and 3) and takes 

 the place of olivine in the present rock -group. It is sedge-like 

 in general shape, and granular in its margin (especially in 

 Fig. 2), };eing fringed with grains of common augite, whose 

 presence becomes strikingly apparent between crossed niçois. 



