178 MR. T. fl. HOLLAND OX EOCK-SPECIiMENS FROM KOREA. 



attempt at the assumption of its normal crystallographic outline, 

 and producinp:, by alternate development of forms, an imperfect 

 example of " babel" quartz. 



Hydrated ferruginous products, titanoferrite, green chloritic 

 material, and secondary quartz, infilling cracks and cavities, have 

 accompanied the distinct devitrification of the magma. The con- 

 tortions and twistings (during the original fiovr of the material) of 

 the patches of unlike chemical composition have given rise to a 

 structure similar to that to which Mr. E-utley has given the name 

 " damascened"*. 



(2) Intermediate Eruptive Bocks. 



The rocks to which Prof. Judd in 1876 gave the name " inter- 

 mediate " are represented in Mr. Gowland's collection of Korean 

 specimens only by members of the series in which plagioclase is 

 the predominating felspathic mineral. These comprise examples 

 of plutonic origin as well as those formed as lavas. 



a. Plutonic. 



Diorite. — An exposure of a fine-grained variety of this rock 

 occurs in a gorge near Yukei, in the province of Chhung-chhong. 

 Scattered through a fine-grained, almost aphanitic groundmass 

 are patches of coarser-grained aggregates of hornblende and fel- 

 spar, forming the " glomero-porphyritic " structure of Prof. Judd. 

 Under the microscope, hornblende, in green and brown crystals, 

 small flakes of biotite, and plagioclase-felspar are seen to be the 

 principal constituents. Occasional granules of quartz, with nume- 

 rous acicular inclusions of apatite, lumps of magnetite, and rarely 

 crystals of zircon, occur as accessories. 



The hornblende in this rock, besides occurring in the form of the 

 numerous small green crystals with an extinction-angle (cAc) of 

 15°, is found more rarely in large crystals of a deep brown colour, 

 with a narrow zone of the green variety around, and in crystallo- 

 graphic continuity with the brown mass in the centre. The cleavage 

 is well marked in the brown crystals, which exhibit a very strong 

 pleochroism from a = straw-yellow to y = deep brown. The strong 

 absorption (y >j3>a) increased the difficulty of making an accurate 

 determinatio7i of the position of extinction ; but 5" was obtained as 

 an average of numerous measurements made from the vertical axis. 

 The properties of the brown variety are thus identical with basaltic 

 hornblende, whilst the green zone exhibits the optical characters of 

 common hornblende. 



The major part of the plagioclase-crystals are zoned by a gradual 

 passage into succeeding layers of different chemical composition 

 from the centres of the crystals to their borders. Distinct, narrow, 

 and irregular fringes with a different extinction -angle sometimes 

 surround the crystals, and were apparently formed after the con- 



* 'The Study of Eocks/ 1879, p. 181. 



