NOTES ON SOME IGNEOUS ROCKS OF JAPAN 639 



A black, thin coating of lava, which crusts the Miyake-jima 

 anorthite and the Naka-io-jima oligoclase-andesine, is not seen 

 on the mineral from Tarumai. The crystals, however, have 

 attached to them a small quantity of light-colored pumice. It is 

 evident that the matrix of brittle pumice separated easily from the 

 crystals and that the semi-solidified lava was not so viscous as in 

 the case of the lava of Miyake-jima and of Naka-io-jima. Also, 

 in some specimens the crystal is in a cavity having well-defined 



Fig. 5. — Well-defined cavity from which the anorthite crystal has been lost. 

 Natural size. 



walls corresponding to the faces of the crystal, with a space about 

 4 mm. in width between the crystal and the lava. The crystal is 

 attached to the walls by slender, needle-like filaments of glass, as 

 shown in the accompanying photographs (Figs. 4 and 5). There 

 may be several explanations of the formation of these cavities, but 

 the writer believes they were formed chiefly by the differential 

 movements of the crystal and matrix when the blocks of lava were 

 ejected in a semi-solidified state. 



The common sizes of the crystals are 10 mm. to 15 mm. in the 



