50 KOTO : NOTES ON THE GEOLOGY 



be of extreme rarity. The laths extinguish light symmetrically 

 but in the contrary direction at an angle of 26°-27°, proving 

 the felspar to be more acidic than its phenocryst. Interstitial 

 space is occupied by a brown glass which contains globulitic 

 and rod- shaped bodies. From the foregoing descriptions, it is 

 evident that the Basalt of the Island of Koto does not properly 

 belong to the category of normal Basalt ivith violet aiigite, present- 

 i7ig the intei'sertal structure, which is so common in the 7'ocks of 

 the Hoko group, already described. Here exclusively monoclinic 

 augite presents the character of diopside. Both the olivine and 

 augite, all being equally corroded, present so great a variation 

 in size from the macroscopic to tlie microscopic dimensions that 

 I could not discriminate the products of the intratelluric from 

 the effusive period of consolidation. The ground-mass, as I have 

 said, is highly ielspathic, and the structure is Andesitic and 

 hypocrystalliue-porphyritic, somewhat resembling a pilotaxitic 

 type. Richness in olivine and paucity in iron ores, as well as 

 globulitically granulated mesostasis make the rock approach 

 to 3, navitic structure [Mg. 4), the only difference being the presence 

 of feldpar-laths in the ground-mass. The rock seems to me to 

 be a lava-flow, consolidated rapidly, accompanied by a brisk 

 liberation of gas from the cooling magma. 



B. HORNBLENDE-ANDESITES. 

 [Fl. Ill, Fig. 5.) 



I liave three specimens of rocks in Torii's collection, belong- 

 ing to the same category. They differ in colour consequent on 

 the various stages of decomposition. A fresh one is greyish and 

 porous, speckled with phenocrysts of hornblende (2 mm. in 

 length). 



