180 MR. T. n. HOLLAND ON ROCK- SPECIMENS FROM KOREA. 



b. Volcanic. 



An interesting series of rocks of the andesitic type occurs in 

 Korea. The specimens I have examined invariably exhibit signs of 

 secondary changes, resulting, in some cases, in a devitrification, more 

 or less complete, of an originally glassy matrix, and passing gra- 

 dually into the structures characteristic of the rocks to which Yon 

 Kiclithoien gave the name of propylites. This change is sometimes 

 accompanied by others due more precisely to subaerial agencies, 

 whereby we find the original alkaline and ferro-magnesian silicates 

 replaced by chlorite, epidote, and calcite ; whilst magnetite has been 

 oxidized and hydrated to the usual ferruginous products. 



Between Indong and Tawon, Mr. Gowland found boulders of a 

 black, glassy-looking, porphyritic andesite in the river-bed. The 

 specimens were associated with stratified sandstones, grits, conglo- 

 merates, and dark slaty rocks of doubtful stratigraphical position *. 

 The andesite, under the microscope, exhibits the most beautiful 

 fluidal structure. Patches of brown, black, and green microliths 

 have been formed in the moving mass. The brown and black 

 microliths, which are seen under the higher powers to be merely 

 minute specks and streaks of ferruginous material, have aggregated 

 into wisp-like radial groups with irregularly curved outlines. These 

 minute bodies closely resemble the minute black rods and granules 

 of magnetite observed by Prof. Judd in a magma-basalt from Gribun, 

 Isle of Mull t. 



Although the matrix appears so beautifully glassy, it is seen 

 under polarized light to be completely devitrified in granular areas, 

 which are apparently formed irrespective of the spherulitic aggre- 

 gates of microliths. 



The most prominent porphyritic constituent is plagioclase, in 

 large crystals very slightly aff'ected by kaolinization. Twinnings, 

 both on the pericline- and the albite-type, are exhibited, and the 

 crystals aie occasionally zoned from change of chemical composition 

 during growth. The grouping together of the felspars occurs in 

 such a manner as to suggest the " glomero-porphyritic " structure 

 of Prof. Judd, thus pointing to their probable allothigenous origin. 

 Occasionally the felspars have been decomposed in the centre, 

 and the cavities infilled with quartz (or, perhaps, albite). On 

 stirring the crushed and sifted rock in a heavy solution of boro- 

 tungstate of cadmium, the majority of the white felspathic grains 

 were just held in suspension when the liquid possessed a density of 

 2'66, thus showing them to be of a composition bordering on 

 andesine and labradorite. This is confirmed by the results obtained 

 in an examination by Szabo's method of flame-reactions. It was 

 found that nine grains agreed closely with the characters given by 

 Prof. Szabo for andesine, whilst six grains showed the characters of 

 labradorite. A silica determination of the very small quantity 



* Dr. Gottsche marks this district in his map as being probably of Carboni- 

 ferous age. 



t ' On the Gabbros, Dolerites, and Basalts of Tertiary Age in Scotland and 

 Ireland,' Quart. Journ. Geol. Sec. vol. xlii. ^886) p. GO, and pi. vi. fig. 7. 



