MR. T. H. HOLLAND ON ROCK-SPECIMENS FROM KOREA. 181 



available gave 50*45 per cent, of silicic acid, the mineral being thus 

 slightly more acid than the compound Ab^ AUj*. 



Secondary quartz with chlorite and acicular apatite infill cavities 

 which present the characteristic shapes of the pyro:ienes originally 

 in the rock. Together with the (]uartz, occasional lumps of mag- 

 netite and, more rarely, granules of yellowish-green epidote occur in 

 these cavities. The rock has a specific gravity of 2-65. 



A porphyritic andcsite occurs about 15 li south-east of Milyang, 

 in Xyong-sang Do. The specimens have an average specific gravity 

 of 2-&2. Under the microscope, the fluidal structure is distinctly 

 shown by the bands of dusty microliths in the brown matrix. The 

 black and brown microliths are so closely felted {Jli/crolithevjllz), 

 that it is impossible to determine the interstitial glass. Throughout 

 the sections, and following the direction of flow, there are grano- 

 phyric bands, which, in places, distinctly exhibit the structure to 

 which Becke, in 1881, gave the name of " centric." From the way 

 in which these bands are drawn out in the direction of flow, they 

 suggest the existence, in the molten material, of bauds of difterent 

 chemical composition ; but there is not, in the present case, any 

 evidence to prove that the structure has been formed in the manner 

 suggested by Prof. Lagorio t> oi' whether it is the result simply of 

 secondary changes. It is worthy of note that the felspathic 

 material thus intergrown with the quartz is invariably decomposed 

 and kaolinized ; and the same structure has, in other cases, been 

 ])roved to be of secondary origin by Prof. E,. D, Irving q:, Prof. J. 

 "W. Judd§, and Miss Raisin |j. 



The plagioclastic felspars are represented in this rock by clear, 

 glassy crystals, which, when examined by Prof. Szabo's method of 

 flame-reactions, agreed in character with oligoclase ; and, on sepa- 

 ration from the crushed matrix by means of a solution of boro- 

 tungstate of cadmium, were found to possess a specific gravity of 

 2-65. In some cases it is easy to prove that the felspars were 

 formed as such before the rock in which they are now found — that, 

 indeed, some of them previously existed as porphyritic constituents 

 of a plutonic rock. In fig. 3, I have represented a case in which a 

 plagioclastic crystal is seen to possess a centre probably much older 

 than the surrounding zones. The central core has been schillerized 

 in four distinct planes, the inclusions being arranged in lines 

 crossing one another at the angles shown in the diagram (fig. 4). 

 There is evidence of corrosion of this central core, and, afterwards, 



* Owing to an accident, part of the bulk analysis of the rock was lost. The 

 silica, however, was found to amount to 0842 per cent. ; in this respect tlie 

 result of the analysis closely resembles that obtained from the dacite-glass 

 of Krakatoa, described by Prof. Judd ('Report of the Krakatoa Committee 

 of the Royal Society,' 1888, pp. 31, 32, and 34). 



t ' Ueber die Natur der Glasbasis, sowie der Krvstallisationsvorgange in 

 eruptiven Magmen,' Tschermak's Min. u. Petrogr. Mittheil. vol. viii. p. 421. 



+ U. S. Geol. Survey, Monograph V. — The Copper-bearing Rocks of Lake 

 Superior (1883), p. 113. pi. xiv. figs. 1, 2, 3, & 4 ; and pi. xv. fig. 4. 



§ Quart. Journ. Geol. Soc. vol. xlii. (1886) p. 73, pi. vii. fig. 8. 



H Ibid. vol. xlv. (1880) pp. 252 and 253. 



