THE ARCH.EAX FOEMATIOX OF THE ABUKUMA PLATEAU. 



227 



Thu habitus of tlie cry.sfal.s of liornbleiide Is to be slender, flat and 

 prismatic, witli the faces oP, y. V ^. , l\x P^ , and cc P ; twins are 

 rave (Fig. 2, Plate XXIII.) The anahtical result o1)tained bv Mv. 

 hlida is as follows : — 



^^O, 45. ,vi 



AI2O3 4. 47 



FeO 8. U'J 



CaO -20. 40 



MgO ]]. 44 



Na/ ) . . '2_ .ji) 



I^.^ 0. 79 



99. S9 



The hornlilende in tlie caught-up granite, like the biotite, shows 

 some peculiar marks entirely f(jreign to the c(3mmon species. Fn 

 tlie jnassive granite the crystals of hornblende are usually idiomorphic 

 while in the caught-up nx-k they are of large dimensions and at the 

 same time allotriomorphic. Inlaying just the same role as the aiiiiite in 

 diabases in the formation of the so-called diabasic structure. In the 

 normal sequence, as is well known from the (observation of Prof. 

 Posenbusch, the crysfalli/Jng-out of the ferro-mngnesian bisiUcates 

 precedes that of the feldspar-group. Here we ha\e some excepti<jual 

 cases, probably arising frcjni the unconnnon circumstances under 

 which the njck reached the solid state. A S(jme\\diat large, all(jtrio- 

 morphic hcjrnblende of a dark-green colour, encloses, as it were, the 

 interstitial mass of basaltic rock, a nuniber of equally allotri(jmorphic 

 crystals of phigioclase oriented in diverse directions (Fiu-. 7^ pj;,te 

 XXITL). The plagioclase ( ? alhite) in tiie present case presents 

 tlie habitus of albite with corroded appearance, occurring usually in 

 large «piantities in the schistose amphib(jle rocks. The h(n-nblende 

 occurs in the form of an opliitic plate with uniform extinction over 



