42 KOTO : NOTES ON THE GEOLOGY 



E. THE TYPE OF THE ANALGIME-BASALTS. 

 (PL JI, Fig. 5.) 



This to the naked eye is macroscopically deep-grey, and fine- 

 granular. Under the microscope it is hypocrystalline and more or 

 less ]3orphyritic, either the xenomorphic olivine or the aggregate of 

 the automorphic augite being the phenocryst, or sometimes both. 

 The texture varies within a wide range, but generally speaking is 

 coarse (Fig. o). The porphyritic elements, however, differ general- 

 ly not much in size from the crystals of the ground-mass, and 

 the mode of arrangement of the several components is graniditic. 

 Plagioclase predominates over augite in quantity ; and magnetite 

 is not plentiful, and completely soluble in HCl. The paucity of 

 iron-ore causes the rock to appear of a grey shade. 



Olivine occurs as a phenocryst in the xenomorphic grains, a 

 few of which have been reduced even to mere flecks through gradual 

 resorption. Cleavages are not noticeable in contrast to other 

 olivines, but in stead of them there are curvilinear cracks, con- 

 forming approximately in their direction to the boundary of 

 resorption. The substance of the olivine is colourless, and usually 

 more or less altered into a greenish or yellowish, fibrous sub- 

 stance (not iddingsitic). Brown decomposition is quite foreign 

 to the olivine of this type. The present olivine seems to belong 

 to a variety rich in magnesia. Phenocrystic pyroxene is scantily 

 present in some, but none in others. The augite is of the typical 

 Basaltic variety, with a violet-brown type, possessing the hour- 

 glass structure, and idiomorphic, flattened on the orthopinacid. 

 It occurs singly or in stellar aggregate. There is no felspar- 

 phenocryst. 



The ground-mass consists of laths of plagioclase, crystals 



