THE ARCHtEAX FORMATIOX OF THE ABUKUMA PLATEAU. 217 



couditioii. The effusive rock that cijiiies here into consideration is 

 mainly granite. It is to be separated into an older, hornblende-bearinü' 

 (H Gr), and a younger, bi otite- bearing (B (jt) variety. They intermingle 

 with one another in such a confused manner as to try the patience of even 

 careful observers ; and many have entirely ignored the heterogeneous 

 origin of the two kinds of massive rock (Fig. 1, Plate XXIV). 



On the way from Ishikawa to a small height where the road as- 

 cends towards Shibukawa, we meet with the continuation of the 

 granite-mass, already referred to in tlie preceding profile. In one spot 

 a mass of di(jritic habitus occurs of tolerably large dimensions. Ex- 

 amined under the microscope, it is found to be mainly composed of feld- 

 spar and needle-shaped liornblende, with a little l)iotite and orthoclase. 

 Macroscopically it appr<jaches in its external aspect to the so-called 

 " needle-diorite " (marked x in rhe sectit)u). Mot fir from this point, 

 another exposure of a siiiula,r kind makes its appearance in a road- 

 cutting. Blackish dioritic patches of various sizes are enclosed in 

 normal hornl)lende-granite, as if a dioritic scum had floated on a 

 granitic paste, and then Ijeen torn asunder by s(jme movement in the 

 semi-fluid mass. Tlie patches seem to have been once a part of the 

 same granitic magma, slightly different in chemical composition (Fig. 

 8, PI. XXIII). These should not be confounded with the caught- up 

 fi-agments of amphibolite in hornblende-granite, or the hornblende- 

 granite in biotite-granite, as the encl(3sed and enclosing rocks 

 in the latter tw(j cases differ both in age and composition, and the 

 patches have angular outlines. 



Just on the to]) of the hill, two large dykes (d) traverse the granite 

 in the E.-W. direction. The material of the dykes is a pegmatic minei-al 

 aggregate with large orthoclase crystals twinned on the Baveno type. 

 Large, opaque biotite, smoky quartz, and muscovite are other ingredients 

 of this aggregate. A dull, brownish beryl is said to have been found in it. 



