ON PYROXENIC COMPONENTS IN CERTAIN VOLCANIC ROCKS. 73 



contains : but the altered portion almost resists fusion. It is found 

 also that the latter is softer, and the specific gravity is less, than the 

 2"lass. The glass and the alteration-product are scarcely attacked by 

 hydrochloric acid. 



The mineralogical composition of the rocks of Tatsumiura and of 

 Miyanoura thus far indicated, would seem to be quite singular. It 

 is probably to be classed as special rank among the glassy form of 

 basaltic rocks. I am indebted to Mr. T. Suzuki, for the chemical 

 analyses given below, of the glassy rocks of Ototojima and Miyano- 

 ura. The analyses were undertaken by Mr. R. Fukuda of the 

 Geological Survey. 



I II 



Si0 2 53-18% 54-44% 



Ah 3 16-18 12-90 



Fe 2 0, 10-30 7-08 



Ca 10-12 5-12 



MgO 6-72 12-75 



K 2 -35 -35 



Na 2 O 1-85 2-06 



loss by ignition .... 1'65 5*54 



100-35 700-24 



I. is the dark spheroidal glass of Sp. G. 2*725, already mentioned, 

 found at Kurose in Ototojima. 



II. is the glassy portion of the rock from Miyanoura, comparatively 

 free from the alteration-product. Sp. G. = 2*75. 



The higher amount of lime in analysis I. probably points to the 

 existence of basic Plagioclase, viz. in the form of the ' Rhombic lamella*.' 

 The high percentage of magnesia in II. is accounted for by the rich de- 

 velopment of the Rhombic Pyroxene, the analysis of which is given in 

 the sequel, while Augite is very scarce. The existence of a large quan- 

 tity of water is also indicated by the large quantity of loss by ignition. 



