THE ARCH^AX FORMATION OF THE ABUKUMA PLATEAU. 253 



vite. Examined from the biise, the mica has a small optic angle, 

 and consequently should be considered as a variety of biotite. The 

 mica is present in fluctuating quantity, and it never takes an import- 

 ant part in the composition of the rock, but still it should be considered 

 as a characteristic ingredient of the aniphibole-schist, since it is entire- 

 ly wantino' in the series next above it. Thouo-h it occurs in meao-re 

 cjuantities, it may be readily recognized on the plane of schisto- 

 sity by its gold-yellow, glilter'nuj lustre. During field work we meet \^'ith 

 several amphibolites, all having nearly the same appearance, but belong- 

 ing to quite different horizons; the presence of the hiotite sliouhl, in tJwse 

 perplexing cases, be taken as a criterion for the identification^ of the 

 amphibolite of this series. We name this rock in the field the hiotite- 

 ainphihidite in order to discriminate it from other kindred varieties. 



The iß) light-green bands in the schist, already referred to, difi'er in 

 many respects from the («) black zones rich in hornblende, by the 

 presence of a light-green augite. Here the grains of feldspar are of 

 minor dimensions, and for the greater part its substance has 

 been changed into an aggregate of a colourless, fine fibrous, confused 

 mass, which between crossed nic(3ls displays highly chromatic, aggre- 

 gate polarization-colours. The intact part of the feldspar appears, 

 therefore, here and there in a sporadic manner within the above- 

 mentioned micaceous mass, which seems to be identical with the 

 so-called pseudophite. This white band, sometimes attaining the 

 breath of 2 or 3 cm., is moreover very dirty, owing to an admixture 

 of titaniferous iron and highly refractive grains of titanite (1 mm.) 

 forming a rim of iron -ore, to which we shall have occasion to 

 return in the sequel. 



The augite (Fig. 12, PI. XXI tl.) already referred to, occurs in 

 this band in great abundance, taking the place of the hornblendic 

 element, and even of the feldspathic component. It is found always in 



