THE AROH^AN FORMATION OF THE ABUKUMA PLATEAU. 279 



compact, dark- greyish rock in which are porphyritically imbedded 

 crystals of hornblende and augite (3 mm.). No other components are 

 visible to the naked eye. Under the microscope it proves to be'pilo- 

 taxitic, consisting of lath-shaped plagvoclase, needles of hornblende, and 

 magnetite, with scanty remains of some colourless basis, which fill up 

 the interstices of the crystalline ingredients. 



The phenocrysts are plagioclase, hornblende, augite, and a 

 zeolitic mineral ; the first is long rectangular or square, occurs 

 in simple crystals, or in simple twins, all extinguishing in oblique 

 directions at more than oO.° In spite of the simplicity of the crystals 

 and the scarcitv of twins, the feldspar may be looked upon as a basic 

 plagioclase. Zonings of the plagioclase are well developed, the 

 central part being often hollow, and partially filled up with a chloritic 

 aggregate ; the peripheral zone is fresli and glassy. The hornblende 

 is greenish-brown ; its forms are never perfect, but eaten away 

 by magmatic corrosive action, and the crystals are in consequence 

 encircled by an aggregate of opacité and needles of augite. These 

 marginal masses are prolonged into long " tails," just as horn- 

 blende is in some porphyrites. Its pleochroism is quite pronounced. 

 Twins of common type are discernible onlif in that shape wliich 

 has been formerly considered as anomalous twins whose plane of contact 

 IMS said to he the prism zc^ 2 (120)} The augite is rare, its 

 habitus is andesitic, consequently its basal section is approximately 

 hexagonal, being bounded by the predominating faces of pinacoids, 

 truncated at the four corners by prismatic feces, and its colour is 

 light-yellow and non-pleochroic. Biotite and olivine are not to be seen 

 in the specimens hitherto examined. This dyke-rock seems to 

 have aldose resemblance to the comptonitic modification of the dioritic 

 lamprophyre. 



1 Becke: Tschermak's Mineralog. unci petrogr. Mitth. Bd. IV, p. 365. 



