TRUE VOLCANOES. 435 



containing glassy feldspar (Leopold von Buch's trachyte 

 proper), are imbedded in the tufas of Monte Somma ; 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 Fhlegrsean 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 sur- 

 face of the earth, which is still, on the whole, so very im- 

 perfectly investigated in a scientifically geological and client 

 ical sense. Modifications in the nomenclature of the asso- 

 ciated minerals, as well as additions to the trachyte forma- 

 tions themselves, are to be expected in two ways, both from 

 the progressive improvement of mineralogy itself (in a more 

 exact specific distinction both with regard to form and chem- 

 ical composition), and from the increased number of col- 

 lections, which are for the most part so incomplete and so 

 aimless. Here, as in all other cases where the governing 

 law in cosmical investigations can only be discovered by a 

 widely-extended comparison of individual cases, we must 

 proceed on the principle that every thing which, in the pres- 

 ent condition of science, we think we know is but a small 

 portion of what the next century will bring to light. The 

 means of early acquiring this advantage lie in profusion 

 before us, but the investigation of the trachyte portion of 

 the dry surface of the earth, whether raised, depressed, or 

 opened up by fissures, has hitherto been very deficient in the 

 employment of thoroughly exhaustive methods. 



Though similar in form, in the construction of their frame- 

 work, and their geotectonic relations, volcanoes situated very 

 near each other have frequently a very different individual 



trachytes of still burning volcanoes (the Peak of TenerifFe and Coto- 

 paxi), and yet at the same time also in the granite and granitite of 

 Schreibersau and Warmbrunn, in the Silesian Riesengebirge (Gustav 

 Rose, in the minerals belonging to the granite group, in the Zeit- 

 schriften d. Deutsch. geol. Gesellsch., 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), has been found erroneous, after many years' investigation, 

 by Dufrenoy (Traite de Mincrahgic, t. iii., p. 399). 



