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



characters of rocks which are known as felsites and curites *. They 

 occur in various parts of the country breaking through the crystal- 

 line schists and granites. 



Between Chhungju and Erambe, Mr. Gowland obtained specimens 

 of a mass of eurite breaking through granite. The rock has a 

 specific gravity of 2'53. Amongst the porphyritic constituents the 

 most conspicuous are quartz, in large transparent crystals, and 

 Carlsbad-twins of ortlioclase. The rock is stained with ferruginous 

 decomposition-products, and contains irregularly-scattered minute 

 cubes of hydrated oxides of iron occurring as pseudomorphs after 

 pyrites. Small flakes of biotite are not uncommon. Prof. Eoth has 

 described a somewhat similar rock as a granite-porphyry from a 

 locality between Paikchi and Ikujang, in the western province of 

 Hwang-hait. 



In the Mungyong pass, the larger quartz-crystals of the eurites 

 exist in well-developed bi-pyramidal forms, the faces being generally 

 considerably etched. Orthoclase, plagioclase, and hexagonal plates 

 of biotite occur in association with the quartz. Iron pyrites and 

 magnetite are present in smaller quantities. In a pass a little 

 to the west of Chhungju Mr. Gowland secured specimens of a 

 similar rock, with a specific gravity of 2*56. Eurites occur also 

 S.E. of Undon, and, like the granites, are frequently traversed by 

 veins of quartz. 



b. Volcanic. 



Corresponding in chemical composition with the foregoing there 

 occur in Korea rocks which, from the structures they exhibit, have 

 had an undoubted volcanic origin, and which have, since their 

 eruption, suffered from a devitrification of their originally glassy 

 magmas, with a production of a secondary felsitic structure, similar 

 to the structure exhibited by many ancient British lavas with 

 which we are familiar from the researches of Rutley, Allport, and 

 Bonney. 



In the province of Chhung-chhong, there occurs to the S.E. 

 of Chhungju an example of this nature associated with granitic 

 rocks. The compact, greenish or greyish hand-specimen shows 

 distinct banding ; and this is confirmed under the microscope by the 

 fluidal arrangement of the raicroliths around the porphyritic con- 

 stituents. With crossed nicols, the field becomes broken up into 

 doubly-refracting patches with no apparent relation to the irregularly 

 distributed masses of nebulous, green, and brown material. Quartz 

 occurs in irregular grains and as bi-pyramidal crystals ; and, in 

 greater abundance, orthoclase in flesh-coloured crystals. The relics 

 of partially decomposed felspars show that plagioclase was by nc 



* The name ' eurite ' has been shown by Messrs. Cole and Jennings (Quart. 

 Journ. Geol. Soc. vol. xlv. (1889) p. 433) to have been employed with scientific 

 precision by d'Aubuisson before such names as ' felsite,' ' quartz-porphyry,' &c. 

 were proposed. I have, therefore, thought its nse preferable to the terms more 

 commonly employed in this country. 



t Op. cit. p. 870. 



