252 



B. KOTO. 



member of the complex of amphibolites and mica-schi«ts ; otlierwise 

 its distribution is not so wide-spread as that of the other rocks. 



B. Titanite-Amphibole-Schists. 



They form manifold alternations with the preceding rock. 

 They are the black, highly crystalline schists with the plane-parallel 

 structure, whose transverse section shows inter banded, thin, light- 

 colom-ed zones, recurring hundreds of times even in a small chip (Fig. 

 1, PI. XXVIl). If we move upward to a higher horizon, the rocks 

 become perfectly schistose, cleaving easily into a papery slab ; and as 

 their texture is not so compact as that of mica-schist, they easily fall 

 into a bluish-black, ashy powder on their weathered surface. 



The («) black zone, when examined under the microscope, is seen 

 to be made up of a bottle-green hornbk'nde, whose pleochroism is 

 distinct, but not so pronounced as that of the brown variety, a, 

 yellowish-white, ß, oily-brown, c, greenish-brown ; the absorption 

 c>ß>a. 



Isometric ijlagiodase-pulygans ^ are present, besides the hornblende, 

 in clear crystals free from any interpositions. All the polygons, so 

 formed from the disposition of neighboring grains, show undulatory 

 extinctions which sweep concentrically from one zone to the other in 

 the crystals, as the section is rotated. Only a few grains (1.3 mm.) 

 are coarselv twinned, with the angle of extinction of 7°-10°. or o°-4° 



■J ' o 



symmetrically with respect to the twinning suture, m that liere we 

 have probably before us an andesine feldspar. 



The lamellge of biotite, with the interleaved bacillar horn- 

 blende, are oily-brown or green, but through weathering have become 

 so entirely decolorized as to be readily mistaken for a typical musco- 



1 Vide ante, p. 2-16. 



