38 KOTO : NOTES ON THE GEOLOGY 



needles may possibly of a felspathic nature and such a structure 

 is termed tarioUiio by Harker/^ though in the present case those 

 circular, whitish spots, called varioles, are wanting. Green, 

 fresh base is here and there also found between the angular spaces. 

 Thin lameHœ of rugged outlines, with violet-brown colour, 

 may be frequently noticed in all of my slides, and they closely 

 resemble tiiose found as interpositions in a hypersthene. I 

 conjecture the minéralogie nature of these plates to be ilmenite. 



6'. THE OPHITIG TYPE. 



{Fl. II, Fl)/. 2) 



This type is represented by a single specimen from Hoko, 

 and Gio-o"' respectively, and two from Haku-sha, though the 

 * lie ' is not known to me exactly ; but it is highly probable that 

 samples are taken from the second sheet which is separated 

 usually from the uppermost columnar flow by an ash bed of a 

 certain tliickness. It is a greyish-black, Anamesitic rock, with the 

 brownish, lath-shaped phenocrysts of plagioclase (4 mm. length). 

 This is the coarsest type of the Hoko Basalts, and is the one rich 

 in plagioclase in comparison with ferro-magnesian silicates ; it 

 seems to have solidified in the lower portion of the lava flow. 



Under the microscope, it shows a porphyritic, hypocrys- 

 talline, diabasic structure {Fig. 2) with the ophitic plates of 

 augite of considerable dimensions, enclosing the laths of plagio- 

 clase which lie in all possible directions. The augite is of a 



1) "The aggregates of felspar-microlites or fibres with fan-like or sheaf-like groupings. 

 They may be closely packed to make up the entire mass of a portion of the rock (Basalt)." 



Petrology for Students.' find. Edit., p. IUI and 201, Fig. 41 A. 



2) Tlie ex:ict locality being Sho-chi-kaku, ('hJfe:^) :»t tlie middle of the iskxnd. 



