THE ARCHJ3AX FORMATION OF THE ABUKUMA PLATEAU. 283 



refracting margins of titanile. Zircon with well-finished forms is 

 also observed. The structure is more or less porphyritic. The part 

 in direct contact with the cheek of granite is typically porphyritic, 

 with feldspar, hornblende, and biotite as large components; while 

 the rest is a crypto-crystalline aggregate of grains of feldspar and 

 quartz, and also foliée <jf biotite, making the ground mass of a beautiful 

 fluid-like structure, seen in the bending of streams, and bands of mica 

 around the larger feldspar-crystals. 



4) TJie coarse porplnjritic modification. — This ditfers entirely from 

 all the preceding rocks in its external appearance, so that any con- 

 founding of this with the other varieties is impossible. It is a 

 typically porphyritic rock with large, rectangular or broad-tabular 

 feldspa.r (3 nun.) imbedded within the greyish ground-mass. The 

 feldspar is zonal ly structured with differentiated optical orientations 

 in each zone. iSonie crystals are simple; a few are coarsely polysyn- 

 thetic with oljlique extinctions of abfjut 14° Avith the suture-line, 

 while about 23° with P/M on the brachypinacoid. Therefore, it 

 corresponds to the mixture of ab, an^, of the labradorite series. 



Minerals of the second generation are also discernible by the 

 naked eye, if carefully examined. They are long-rectangular, polysyn- 

 tlietic plagioclase, devoid of zonal shells, and a greenish prismatic 

 hornblende with combination of the })yramid, prism, and brachy- 

 pinacoid. The hornblende is greenish-brown oc bluish-green, and 

 pleochroic. A dirty-green biotite is found with sagenitic rutiles in 

 apparently fresh lamella?. 



The general ma^s in which the components of the earlier genera- 

 tions are found, consists of a crypto-crystalline aggregate of grains 

 of feldspar and quartz, and also shreds of a green biotite. Acces- 

 sories are rounded apatite, and titaniferous iron with the margins of 

 leucoxene. 



