46(5 COSMOS. 



In the present lavas, on the contrary, the atigites predomi- 

 nate and the leucites are on the whole very scarce, although 

 the lava-stream of the 2 2nd April, 1845, has furnished them 

 in abundance. 83 Fragments of trachytes of the first division, 

 containing glassy felspar (Leopold von Buch's trachyte 

 proper), are imbedded in the tufas of Monte Sornma ; they 

 also occur detached in the layer of pumice which covers 

 Pompeii. The leucite-ophyr- trachytes of the sixth division 

 must be carefully distinguished from the trachytes of the 

 first division, although leucites occur in the westernmost 

 part of the Phlegraean fields and on the island of Procida, 

 as has been already mentioned." 



The talented originator of the above classification of vol- 

 canoes, according to the association of the simple minerals 

 which they present, does not by any means suppose that he 

 has completed the grouping of all that are found on the 

 surface of the earth, which is still on the whole so very 



83 See Pilla, in the Compfes rendus de VAcad. des Sc., t. xx, 1845, 

 p. 324. In the leucite-crystals of the Rocca Monfina, Pilla has found 

 the surface covered with worm-tubes (scrpulcu), indicating a submarine 

 volcanic formation. On the leucite of the Eifel, in the trachyte of the 

 Burgberg near Riedeu, and that of Albano, Lago Bracciano, and Bor- 

 ghetto, to the north of Rome, see above, page 32, note 93. In 

 the centre of large crystals of leucite, Leop. v. Buch has generally 

 found the fi-agment of a crystal of augite, round which the leucite- 

 crystallisation has formed, " a circumstance which, considering the 

 ready fusibility of the augite, and the infusibility of the leucite, is 

 somewhat singular. More frequently still are fragments of the funda- 

 mental mass itself enclosed like a nucleus iu leucite-porphyry." Olivine 

 is likewise found in lavas, as in the cavities of the obsidian, which I 

 brought from the Cerro del Jacal in Mexico (Cosmos, vol. i, p. 268, 

 note J), and yet, strange to say, also in the hypersthene rock of 

 Elfdal (Berzelius, Sechster Jahrexbericht, 1827, s. 302), which was 

 long considered to be syenite. A similar contrast in the nature of the 

 places where it is found is exhibited by oligoclase, which occurs in the 

 trachytes of still burning volcanoes (the Peak of Teneriffe and Cotopaxi), 

 and yet at the same time also in the granite and granitite of Schreiber- 

 FHU and Warmbrunn in the Silesian Riesengebirge (Gustav Rose, in 

 the minerals belonging to the granite-group, in the Zeitscliriften d. 

 Deutsch. geoL Geselhch., zu Berlin, Bd. i, s. 364). This is not the case 

 with the leucite in the Plutonic rocks, for the statement that leucite 

 has been found disseminated in the mica-slate and gneiss of the 

 Pyrenees near Gavarnie (an assertion which even Hauy has repeated) 

 lias been found erroneous, after many years' investigation, by Dufreuoy 

 (Traits de Mincralogie, t. iii, p. 399). 



