MR. T. n. HOLLAND ON -ROCK-SPFXIMENS FROM KOREA. 3 83 



^[agnotite, in cubic cr5'stals, is scattered throughout the rock. 

 Tlie original fcrro-raagnesian silicate seems, from the shapes of the 

 luasses of secondary products, to have been a pyroxene. It is now 

 ic'j>hiced by viridite, magnetite, granular epidote, and quartz. 



Fragments of other andesites are caught up in the rock. They 

 exliibit a greater development of lath-shaped crystals, and, from the 

 amount of ferruginous material present in them, are apparently 

 derived from rocks more basic in character. 



Specimens of a vesicular andesite were found in the same district, 

 a little nearer Milyang. The phigioclase-felspars, frequently almost 

 wholly converted into epidote, are arranged in a direction parallel 

 to the flow of the rock. The vesicles are frequently lined with 

 opaline silica, and sometimes filled with brightly-polarizing radial 

 aggregates of zeolites. The opaline silica is seen under high powers 

 to be composed of minute, globiform bodies, similar to the structures 

 figured and described by Vogelsang from the so-called quartz- 

 trachyte of Hliniker Thai, Schemnitz *. The opal is stained green, 

 and frequently drawn out into streaks. In opal similarly occurring 

 in a hornblende-dacite from Santorin, MM. Fouque and Michel- 

 Levy suspected the green staining to be due to the presence of 

 silicates of iron and magnesia f. 



Small plates giving dull polarization- colours suggest the presence 

 of tridymite ; but although many of these are hexagonal in outline, 

 I have found no definite cases attributable to the characteristic fan- 

 like twins described by Yom Kath. Silica has crystallized in this rock 

 also as secondary quartz. The relics of the porphyritic felspars give 

 extinction-angles which indicate a composition more basic than that of 

 the small, lath-shaped crystals exhibiting binary twinning. Assuming 

 the latter crj'stals to be developed along the edge oP : oo P oo (001 : 

 010), as pointed out by Michel-Levy J, then the microliths in the 

 present case have a chemical composition closely agreeing with that 

 of oligoclase. They are more acid, and probably belong to a later 

 period of consolidation than the larger crystals, which are thus por- 

 phyritic in the sense in which that term is employed by E,osenbusch§. 

 The porphyritic felspars are greatly kaolinized, and have frequently 

 been partially converted into yellowish granular epidote, which 

 occurs in some cases as isolated granules, and at other times is 

 spread out in patches, extending to the borders of the felspar- 

 crystal, but never exceeding such limits. These facts agree with 

 Mr. llutley's suggestion as to the formation of epidote from kaolin, by 

 the secondary action of carbonates of lime and iron in solution j|. In 

 a rock like this, vesicular in structure and abounding in zeolites and 

 hydrated ferruginous products, the conditions could not have been 



* ' Die Kryst^lliten,' 1875, p. 139, pi. xv. fig. 1. 



t 'Mineralogie micrographique,' 1879, p. 179, pi. xviii. 



I Annales des Mines, ser. 7, vol. xii. p. 451. 



§ ' Ueber das Wesen cler kornigen and porphyrischen Siructur bei Ma?sen- 

 gesteinen,' Neues Jahrbuch f. Min. &e. (1882) p. 13. 



II 'On the possible origin of some Epidosites,' Quart. Joura. Geol. Soc. 

 vol. xliv. (1888) p. 740. 



