376 H. S. Washington — Tschian Trachyt 





Si0 2 60-77 



A1 2 3 19-83 



Fe 2 Q 3 4-14 



FeO _ 2-43 



MnO Trace. 



CaO 1-63 



MgO 0-34 



Na 9 4-90 



K 2 6-27 



P 2 5 ... 0-002 



Ignit. 0-24 



100-55 



Under the microscope the largely predominating, clear 

 glass basis shows a rather light coffee color. The phenocrjsts 

 of sanidine are clear and not generally twinned. Inclusions are 

 uncommon and consist of glass, magnetite and an occasional bio- 

 tite, zircon or apatite. They are quite constantly surrounded 

 by a narrow zone of deep brown glass, in which lie small 

 prismatic microlites of orthoclase, generally at right angles 

 to the domal and basal planes of the crystal. Besides 

 these large feldspars are seen a few smaller phenocrysts of 

 magnetite, green augite and biotite. 



Scattered through the basis, often sporadically, but generally 

 in clusters or irregular streaks, are small prismatic orthoclase 

 crystals, which seldom exceed 0'05 mm in length, and are gen- 

 erally only 0*Q2 mm long. These are usually simple, twinning 

 not having been seen, and only rare cases of forked forms or 

 those brushy at the end. 



They also are surrounded by a border of darker substance, 

 or lie in large irregular patches or long streaks of this, where 

 they occur together. This substance has no appreciable action 

 on polarized light and shows but few signs of definite struc- 

 ture, though a tendency toward the formation of small 

 spheroidal masses is noticeable under high powers, and is 

 rendered more evident by the presence of a little globulitic 

 dust. 



When we examine the next less glassy specimen (No. 553) 

 we find that, while the phenocrysts remain similar in general 

 characters, the narrow dark border has disappeared and the 

 feldspar needles have increased in number, giving rise to a 

 fringe of fine orthoclase needles, separated from each other by 

 extremely minute trichites and globulites. This fine narrow 

 fringe is especially prominent on the ends of crystal sections, 

 cut perpendicular to the plane of symmetry, the sides, as a 

 rule, showing no such borders, but ending cleanly against the 

 groundmass. In one case a rather small and narrow sani- 



