140 Mr Kupffer's Account of the Russian Steppes, fyc. 



glassy feldspar, and small scales of black mica. This lava re- 

 sembles much the trachyte of Elbrouz in its composition, dif- 

 fering from it only in the porosity of its mass. The cavities 

 or vesicles are elongated in a determinate direction, and ar- 

 ranged in strata, a very distinctive character, which seems to 

 belong: to all these melted masses. This character sometimes 

 vanishes ; the mass becomes more compact, and its colour more 

 sombre. The rock is thus a true trachyte with a base of ob- 

 sidian, which perfectly resembles that of the summit of Elbrouz. 

 Other specimens have a pale violet colour, with the aspect 

 of wacke, enveloping also crystals of glassy feldspar and scales 

 of mica. I have also met with specimens in which very thin 

 strata of black trachyte, with a base of obsidian, alternate with 

 a red and very porous scoria. 



The porous feldspar lavas with crystals of glassy feldspar, 

 which I have mentioned, rarely envelope portions of hyaline 

 quartz. These portions are very transparent, and often crack- 

 ed, so that they separate into several grains when touched with 

 the point of a knife. These grains, however, preserve the or- 

 dinary hardness of quartz. These gray lava sometimes become 

 so porous, that they form very light masses, decomposed at 

 the surface, white and similar to pumice-stone, which they re- 

 semble also in roughness. The debris of this rock forms accu- 

 mulations, which we have several times observed in returning 

 from Elbrouz. 



There still remains to be described a particular rock which 

 I have frequently met with in returning from the summit of 

 Elbrouz. This rock consists of a grayish-white feldspar ce- 

 ment, enveloping grains of hyaline quartz, small needles of am- 

 phibole, and scales of black mica. The cement crystallizes at 

 some points, and then presents the structure of glassy feldspar. 

 This rock exhibits the same stratiform structure which belongs 

 to all the lavas of the Caucasus, and which gives to all these 

 rocks the appearance of having flowed in a certain direction. 

 On the banks of the upper Malka we still meet with large 

 blocks of red and green jasper enveloped in the same black vi- 

 treous trachyte which we have above described. 



In all our journies in the Caucasus, I have met with gra- 

 nite only in the hollows excavated by the rivers ; but we know 



