ON ITS EXTERIOR. VOLCANOES. 441 



ago as 1798, that they often form round a crystal of 

 augite, and had described this in an excellent me- 

 moir. ( 621 ) The augite crystal around which, according 

 to the remark of that great geologist, the leucite forms 

 is seldom wanting, but yet appears to be occasionally 

 replaced by a little nucleus or morsel of trachyte. The 

 unequal degree of fusibility between the kernel and 

 the surrounding leucite mass opposes some chemical 

 difficulties to the explanation of the mode of formation 

 within the envelope. Leucites, in part detached, ac- 

 cording to Scacchi, and in part intermixed with the lava, 

 were extremely abundant in the eruptions of Vesuvius 

 in 1822, 1828, 1832, 1845, and 1847. 



Olivine. 



Olivine is very frequent in the old lavas of Vesu- 

 vius ( 622 ), particularly in the leucite-ophyrs of the Somma ; 

 in the Arso of Ischia, in the eruption of 1301, mingled 

 with glassy felspar, brown mica, green augite, and mag- 

 netic iron ; in the Eifel volcanoes which have sent forth 

 lava-streams (e. g. in the Mosenberg, west of Mander- 

 scheid) ( 623 ) ; and in the south-east part of Teneriffe, 

 in the lava-eruption of Gruimar in 1704; but I have 

 sought for it zealously, but in vain, in the trachytes of 

 the volcanoes of Mexico, New Granada, and Quito. 

 Our Berlin collections contain from only four volcanoes, 

 Tungurahua, Antisana, Chimborazo, and Pichincha, 

 68 pieces of trachyte, 48 of which were brought home 

 by myself, and 20 by Boussingault.( 624 ) In the basaltic 

 formations of the New Continent, olivine occurs as fre- 

 quently near augite as in Europe ; but the black basalt- 



