Washington — Igneous Rocks from Eastern Siberia. 183 



ent only to a limited extent. There may have been glass in 

 the rock originally, but, if so, devitrification has obscured it. 



As no analysis was made of this rock its exact relationships 

 cannot be discussed. It is evident, however, that it is rather 

 more siliceous than the trachytes proper, and probably occu- 

 pies in this respect a position intermediate between them and 

 the rhyolites. The character of the feldspars indicates that it is 

 decidedly alkalic, and with potash and soda probably in about 

 equal amount. 



Obsidian. — Three of the specimens are of flow rocks, highly 

 glassy in character, but much devitrified, and offering little of 

 interest. 



One of these is purplish gray, with small yellowish spots and 

 chains of what megascopically look like spherulites, but which 

 the microscope shows to be at present, in great part, irregular 

 quartz grains. A few are still true spherulites, and it is prob- 

 able that they were all so originally. The groundmass, which 

 was formerly glassy, is now thoroughly devitrified to a crypto- 

 crystalline aggregate, apparently of feldspar with subordinate 

 quartz. Ferromagnesian minerals are practically absent. 



Another is a glassy breccia, much decomposed, of a general 

 reddish brown color. Irregular fragments of a feldspathic 

 rock, sometimes itself glassy, and often carrying aegirite, are 

 cemented by a ferritic glass, which shows well marked flow 

 structure. 



The third is a dense black rock, almost wholly composed of 

 devitrified and dusty glass, with fragments of quartz and of 

 alkali-feldspar. 



Monzonite. — The last rock to be described differs so much 

 in a chemical- and mineralogical way from all the others that it 

 is highly probable that it belongs to a distinct rock series, 

 though it also comes from Iskagan Bay. It is also the only 

 one which was almost certainly of intrusive character. 



It is of medium grain and granitic texture, composed of very 

 white feldspar, with a little quartz, and quite abundant stout 

 prisms and anhedra of black hornblende, though the amount 

 of this is very much less than that of the other minerals, prob- 

 ably constituting from 20 to 25 per cent, of the whole. 



In thin section the feldspar is seen to predominate very 

 largely over all the other constituents, and to belong to two 

 species. One of these is a finely striated plagioclase in partly 

 automorphic individuals, which Levy's method shows to be a 

 labradorite of the composition AbjAn^ Along with this, but 

 xenomorphically developed as interstitial masses, is an alkali- 

 feldspar which, judging from the optical characters, is a pre- 

 dominantly potassic orthoclase. Some of this latter is cloudy 



