THE, ARCH^AN FORMATION OF THE ABUKUMA PLATEAU. 281 



fracture. Under the microscope nearly the half of the whole bulk 

 is seen occupied by granophyric bundles which consist of fibrous 

 feldspar and quartz arranged in a divergent-radial manner around a 

 feldspar-crystal, so as to form a spherulite. Between crossed niçois the 

 spherulite is divided into imperfect sectors which sweep along one after 

 another as the section is rotated on the stage. A lew plates of horn- 

 blende and lamellae of muscovite are irregularly intermixed with the 

 granophyric mass, from which it may l)e inferred that the spherulitic 

 bundles came into existence after the crystallization of the bi-silicates. 

 The formation of these bundles is probably the effect of an unequal 

 cooling, or they may have lieen formed secondarily in a manner 

 somewhat similar to that of devitrification l^y molecular rearrangement 

 and differentiation of an unindividualized mass.^ The remainder of 

 the rock consists of a cryptocrystalline aggregate of quartz and feldspar, 

 muscovite and green hornblende; most of the colourless components 

 becoming visible only on applying crossed niçois. Copper pyrite is 

 present. 



2) The dark and fine-granular dyke rock has a few feldspar- crystals 

 as a porphyritic element. îio other minerals are discernible by the 

 naked eye. Under the microscope the porphyritic feldspar is found 

 beautifully zoned with different optical orientations in the centre and 

 the periphery, having an extinction-angle of more than !20° with the 

 twinning sutures; the decomposition begins in some from the core, 

 whereas in others just the reverse hapj)ens. The minerals of the 

 second generation are the bluish-green, fibrous hornblende, and 

 zonally structured, long-rectangular or square-shaped feldspar; while 

 the general mass is made up of needles of hornblende and lamellae of 

 tea-green biotite, lath-shaped feldspar, and grains of quartz. The 

 structure is holocrystalline-porphyritic. Accessories are apatite, octa- 



1 Vide ante p. 235. 



