34 KOTO : NOTES ox THE GEOLOGY 



pecially accumulates near the margin of the secretionary mass, 

 the augite being sometimes arranged along the linear common 

 base, with the free ends of the crystals toward the interior. These 

 phenomena indicate that the lava' had consolidated in a quiet 

 state. 



The relative proportion of augite and plagioclase is also vari- 

 ous, and in the cases where the former outweighs the latter, the 

 olivine increases in its quantity and comes also in the ground- 

 mass, as a product of the crystallization of the effusive period ; 

 and at the same time the texture of the rock becomes finer. 

 If, on the other hand, the plagioclase becomes predominant over 

 the augite, then, the texture gets coarser and more crystalline, 

 and the distinction between phenocrysts and ground-mass is not 

 then commonly well maiked. Apatite and ilmenite seem to occur 

 in the latter variety only, the ilmenite is sometimes transparent 

 with a deep brown colour. 



The only mineral that serves as the phenocrijsi is olivine. 

 Its forms are various, owing to the various degrees of resorption. 

 Most have partial crystallographic faces with dee|D indentations 

 of corrosion, and a drop-like black iron-ore and feUpars were 

 formed in those spaces. Sometimes the act of corrosion has ad- 

 vanced so far that there remain but patches as the relics of a 

 large crystal, and^the eating away oi tlie body by the magmatic 

 menstruum proceeds always from the lateral pinacoids. As usual, 

 the crystals of the olivine are not fresh ; but the routine of change 

 is the same in all. They become fibrous and lamellar, parallel to 

 one of the lateral ^linacoids, the altered portion being yellow or 

 brown, according to the degrees of transformation. The mode 

 of change is similar to iddingsitizalion {Figs. 1 and 2). 



The (jroimd-)nass consists, first of all, of the crystals and 



