18 KOTO : NOTES OX THE GEOLOGY 



bably later Tertiary, and of its insular position, waves gnawed 

 the ground in time, finally reducing the once wide volcanic 

 field into the ruins of islands, as we see at present. Tt is not 

 easy to know the former extent, and the ancient surface 

 feature, of this lava-flat ; but, generally speaking, the relief 

 becomes higher as we go southwards from one to the other 

 in the islands of the jS"orthern group. 8aitô recognises, as I 

 have already said, three lava-flows in the Northern group, 

 viz., the uppermost or youngest being of columnar, the middle 

 porous and vesicular, and the lowest also partly vesicular, and 

 Agglomeratic. After the comparative study of the Basaltic rocks, 

 to which the eflusives exclusively belong, several important facts 

 are brought to light, and now I am able to say, that the young- 

 est flow^ (a, b and ? d types) contains the iddingsitized olivine, at 

 least in one type, and violet brown titan-augite ; the second 

 (c and ? d types) the brown augite, olivine sometimes lacking, 

 being often replaced by hypersthene ; and the third (e type) the 

 analcime-bearing. I w'ill record first my observations on the 

 component-minerals, and then giye the special descriptio?i of rocks. 



A. Component-minerals of Basalts. 



OLIVINE. 



Olivine is rarely automorphic, but mostly xenomorphic, 

 being the remains of resorption by magma. The olivine in the 

 Basalts of the Hôko Islands seems to be of several varieties. 

 Automorphic ones show^ vivid polarisation-colours, and alter 

 usually into some red minerals. The xenomorphic type shows com- 

 paratively a low degree of polarisation, and suffers deep corrosion, 



