32 KOTO : NOTES ON THE GEOLOGY 



In my few slides of Basalts, bearing the iddingsUised olivine, 

 ilmenite seems to he ivanting, though the rocks approach to a 

 Doleritic type, being replaced by magnetite. I cannot say 

 positively that this rule holds true for all the iddingsite- bearing 

 Basalts. 



The magnetite is, on the other hand, the prevailing ore in 

 the compact Basalt, and in the Limburgitic type, in the form 

 of isometric crystals and dust, occurring either in the general 

 mass, or else enclosed in augite and olivine. The face of the crystals 

 shows a metallic lustre with a tinge of blue by reflected light. 

 The dust is sometimes peripherally altered into a blood-red 

 iron-glance. A slide made of a chip from Hoko, was digested 

 in HCl with the addition of KI ; and then the black ore, 

 therein contained, was entirely removed, and the solution not 

 coloured when tested with tin-foil, proving thus the presence of a 

 pure magnetite. As it is already stated above, the magnetite- 

 rich, compact type seems to make up the upper portion of the 

 thick flows of the Hoko Basalts. 



In the Anamesitic type from the islet of Gio-o, we find beauti- 

 ful networks of the skeleton-crystals of magnetite in a devitrified 

 mesostasis within the polygonal spaces between crystals. They 

 are the dodecahedric dendrites, consequently the skeletons inter- 

 sect each other at the angles of 60° and 120°, and are said to 

 consist of garnetohedrons. They all go into solution by treating 

 with HCl. Morozewics^^ tells us that the spire and filigree-work 

 of the skeleton magnetite, crystallising out of the magma rich in 

 iron oxides, consist of minute octahedra, arranged rectilinearly 

 in the direction of the crystallographic axes with secondary and 



1) ' Experimentelle Untersuchungen über die Bildung der Minerale im Magma/ 

 T&chermak's MUthcilungen, 18, 1898, S. 90. 



