MR. T. n. HOLLAND ON ROCK-SPECIMENS FROM KOREA. 179 



solidation of tho rock, in the same manner as was shown by Prof. 

 Judd to be the case with tlie porphyritic plaj^ioclasc in the labra- 

 dorito-andcsite ot iJiin da (jihaoithe, in the Isle of Mull *. 



Near SoUul, a small town to tho north-east of Soul, occurs a 

 mcdium-graiued, friable rock, with a specitic gravity of 2-82, in 

 which quartz, felspar, black mica, hornblende, and sphene are 

 recognizable. In addition to these, the microscope reveals the 

 presence of such accessories as apatite, in bacillar crystals, zircon, 

 and magnetite. The brown " uniaxial " mica and hornblende are 

 found intimately iutergrown ; the latter sometimes exhibiting its 

 idiomorphic outlines. Cross-sections of such hornblende crystals 

 sometimes show, in addition to the more usual prismatic and clino- 

 pinacoidal faces, the traces of the orthopinacoidal plane. The pleo- 

 chroism is n = straw-colour, /.d= grass-green, y = bluish green, and 

 the extinction-angle varies from 13° to 15*^. Twinning on the 

 plane 100 (x P x) commonly occurs. The biotite seems to be 

 more generally included in the hornblende, the latter being some- 

 times quite subordinate in quantity, and occasionally existing 

 merely as irregular and discontinuous fringes to the mass, but which 

 may be recognized, from the same physical orientation over largo 

 areas, as fragments of one crystal. 



Of the felspars, plagioclase predominates. The crystals are fre- 

 quently composed of zones having different extinction-angles, and 

 in the centres are frequently kaolinized over a definitely marked area, 

 the surroundirg zones being apparently untouched by weathering 

 agents. These centres are so well marked, and so deeply decom- 

 posed, that one might well suspect their allothigenous origin, whilst 

 the remainder of the crystal might have been produced during the 

 general and final consolidation of the rock. 



Quartz in fair quantity exists as crystals, which are allotrio- 

 morphic to the other constituents of the rock. 



The presence of sphene is conlirmed by the characteristic reactions 

 for titanium obtained during a chemical examination of the rock. 



Evidence of the pressure to which this rock has been isubjected is 

 aftbrded in the bent twin-planes of the plagioclastic felspars, and 

 the *' undulose " extinctions of the quartz crystals. 



Mr. Gowland obtained, in a stream tt.E. of Yuko, pebbles of 

 a rock somewhat similar in mineral composition, but coarser in 

 crystalline character. Owing to the depth of the soil, exposures 

 were rare in this district ; and consequently their absence prevented 

 the determination of the geological characters of the area. Mr. Gow- 

 land was compelled at this part of the journey to contend with 

 somewhat exceptional difficulties from the hostility of the natives, 

 who several times attempted to stone him and his party, until, at 

 Taiku, he was rescued by the guards sent by the Governor, and 

 thus accompanied throughout the route to the coast. 



* Quart. Journ. Geo!. See. vol. xlv. (1880) p. 178. 



