184 MR. T. II. HOLLAND ON R0CK-SPEC131ENS FROM KOREA. 



unfavourable to sucli a change being brought about in a kaolinized 

 felspar. 



The ferro-magnesiau silicates which originally existed in the rock 

 have been destroyed ; but the casts of secondary products bear the 

 form of hornblende crystals. Around each crystal there is a zone 

 of opacite (magnetite ?) granules, so commonly occurring in horn- 

 blende -andesites — for example, in that of Altsohl, Hungary. Some- 

 times, however, the whole crystal has been destroyed. If these 

 borders be due to decomposition subsequent to consolidation, one 

 might reasonably expect that the changes would extend to the 

 centres of the crystals in a case where alteration has been so 

 thorough. It seems more likely that the dark zones have been 

 produced by the caustic action of the magma before consolidation, 

 and after the relief of pressure in the rock, as in the manner 

 suggested by Zirkel *, and confirmed by the experiments of Becker f. 



(3) Basic Igneous Rocks. 



The basic rocks are especially well represented in the southern 

 provinces, where they break through the crystalline schists, 

 palaeozoic rocks, and granites. The same succession has been 

 observed by Dr. Gottsche in several parts of the country, and Capt. 

 Basil Hall mentions a " whin" dyke cutting through the micaceous 

 schists on Hutton's Island (lat. 36° 10' K, and long. 126" 13' E.±). 



The basic rocks which have been collected by Mr. Gowland are 

 all tine-grained in texture, and have, in nearly all instances, under- 

 gone considerable alteration ; they may be classed as altered 

 varieties of dolerites and basalts. There is a gradual passage 

 represented from the intermediate to the basic group ; and in some 

 cases it becomes a matter of considerable difficulty to distinguish 

 augite-bearing andesites from the basalts. 



Although the examples of basic rocks from the southern provinces 

 are nearly all altered to a considerable extent, there occur, to the 

 north-east of Soul, plateaux of basaltic lavas of comparatively recent 

 date. These are mentioned by Dr. Gottsche and by Mr. W. II. 

 Carles §. Besides these inland occurrences, Charles Gutzlaff, in 1834, 

 recorded the occurrence of columnar " bay-salt " on the west coast 

 near Changsan ||. 



* Ueber d. Kryst. Gesteine liings d. 40-Breitgrade in N. A,' Bericht k. 

 Sachs. Gesellsch. Wissensch. (1877) p. 181. 



t " Ueber die dunklen Uinrandungen der Hornblende und Biotite in den 

 massigen Gesteinen." 



Prof. Judd, in his paper on the ' basic ' rocks of Scotland and Ireland 

 (Q. J. G. S. vol. xlii. (1886) p. 79), refers to the crystallization, in the effusive 

 basic rocks, of lar^e quantities of magnetite, which must, in the deep-seated 

 types, have crystallized out in the ferriferous enstatites. In these andesites, 

 therefore, there might be, on eruption and relief of pressure, a corrosion of the 

 ferriferous silicates, previously formed under greater pressure. 



I Op. cit. Appendix, p. cxxv. 



§ Proc. Eov. Geogr. Soc. vol. viii. (1880) p. 3C5e^ seq. 



II Op. cit. p. 233. 



