VOLCANOES. 



71 



Having now, in our physical delineation, por- 

 trayed the general structure and dynamic ac- 

 tivity of volcanoes, we have still to cast a 

 glance at the material diversity of their prod- 

 ucts. The subterraneous forces separate old 

 combinations of elements, in order to bring 

 about new combinations ; they farther, and at 

 the same time, put the matters transformed or 

 changed into motion, so long as they are dis- 

 solved by heat and moveable. The solidifica- 

 tion of the tenaciously or more limpidly fluid 

 and moveable mass, under different degrees of 

 pressure, appears to be the principal cause de- 

 termining differences in the structure of Plu- 

 tonic and volcanic rocks or mineral species. 

 The mineral mass which has flowed in a liquid 

 state from a volcanic opening — a molten min- 

 eral spring — is called lava. Where several 

 streams of lava have encountered and several- 

 ly restrained each other in their course, they 

 spread out and fill extensive basins, where they 

 cool into stratified beds. These few points 

 comprise the whole of the general features in 

 the productive activity of volcanoes. 



Minerals which merely break through a vol- 

 cano often remain enclosed in the products of 

 its igneous activity. I have, for instance, seen 

 angular masses of syenite, rich in felspar, con- 

 tained in the black augitic lava of the Mexican 

 volcano, Jorullo ; but the masses of dolomite 

 and granular limestone, which contain beauti- 

 ful druses or cavities lined with crystallized 

 minerals — vesuviane and garnets, mejonite, 

 nepheline, and sodalite — are not ejections of 

 Vesuvius : " they rather belong to a very ex- 

 tensive formation, tuff-strata, older than the 

 upheaval of Sornma and Vesuvius, and are 

 probably products of submarine volcanic influ- 

 ences, at great depths below the surface"(^^^)- 

 Among the products of our present volcanoes 

 there are five metals : iron, copper, lead, arse- 

 nic, and selenium, discovered by Stromeyer in 

 the crater of Volcano. Through the smoking 

 fumaroles,' the chlorides of iron, copper, lead, 

 and ammonium, are sublimed ; iron-glance(^^=^), 

 and common salt (the latter often in large quan- 

 tities), are seen filling veins in recent streams 

 of lava, or covering fresh fissures of the cra- 

 ter's edges. 



The mineral composition of lavas differs ac- 

 cording to the nature of the crystalline rock of 

 which the volcano consists ; according to the 

 height of the point at which the eruption takes 

 place — as it is near the foot of the mountain, 

 or in the vicinity of the crater ; and according 

 to the temperature of the interior. Vitreous 

 volcanic products, obsidian, pearlstone, or pum- 

 ice, are entirely wanting in some volcanoes, 

 and in others are only ejected from the crater 

 itself, or from some considerably elevated point. 

 These important and complex relations can 

 only be ascertained by careful crystallographic 

 and chemical researches. My companion in 

 my Siberian journey, Gustavus Rose, and after 

 him, Herman Abich, have begun, with much 

 acumen and success, to throw clear light upon 

 the compact texture of such a variety of vol- 

 canic minerals. 



The greater portion of the vapour that rises 

 is pure steam or watery vapour. Condensed 

 and flowing away as a rivulet, it is used by the 

 goatherds of the island of Pantellaria. The 



stream which was seen flowing from a latera 

 fissure in the crater of Vesuvius on the morn- 

 ing of the 26th of October, 1822, and was long 

 regarded as hot water, was found by Monticelli 

 to be dry ashes, which poured forth like drift- 

 sand ; it was lava ground to dust by attrition. 

 The appearance of ashes, however, which dark- 

 en the air for hours, and even for days, and 

 which, by adhering to the leaves, become so 

 destructive to vineyards and olive trees, in 

 their columnar ascent, borne up by vapours, 

 indicate the termination of every great erup- 

 tion. This is the magnificent spectacle which 

 I the younger Pliny describes in the celebrated 

 j letter to Cornelius Tacitus, and which he com- 

 I pares, in point of shape, to a lofty-branched 

 j and shady pine tree. What has been descri- 

 i bed as flame in the eruption of ashes, is cer- 

 tainly not, any more than the light of the glow- 

 ing red cloud that floats above the crater, to 

 be ascribed to hydrogen gas on fire. It is rath- 

 er the reflection of light from the upheaved 

 molten masses ; sometimes, too, it may be the 

 light from the depths of the fiery gorge cast 

 upon the ascending vapours and reflected by 

 them. But as to what those flames may be, 

 which have been occasionally seen ever since 

 Straho's time during the activity of volcanoes 

 on the coast, and that have risen from the bo- 

 som of the sea immediately before the uphea- 

 val of a volcanic island, I do not pretend to de- 

 cide. 



If we are asked what it is that burns in vol- 

 canoes, what it is that produces the heat which 

 melts and mixes the earths and metals, and 

 even imparts an elevated temperature, for ma- 

 ny years, to streams of lava of great thick- 

 ness ?(^") there is always the presumption 

 that, as in the case of the coal fields which 

 catch fire and go on burning, volcanoes. must 

 necessarily be connected with the presence of 

 certain substances calculated to support com- 

 bustion. According to the various phases of 

 chemical science, we have had bitumen, iron 

 pyrites, the moist contact of finely divided sul- 

 phur and iron, pyrophoric substances, and the 

 metals of the alkalies and earths, assigned as 

 the cause of volcanic phenomena in their high- 

 est intensity. The great chemist to whom we 

 are indebted for our knowledge of the most 

 combustible of the metallic substances, Sir 

 Humphrey Davy, has himself renounced his 

 bold chemical hypothesis in the last volume 

 he published — " Consolations in Travel, and 

 the Last Days of a Philosopher" — a work that 

 excites painful feelings of regret in the mind 

 of the reader. The great mean density of the 

 earth (5-44), compared with the specific gravi- 

 ty of potassium (0 865), of sodium (0972), and 

 of the metals of the earths (12), the absence 

 of hydrogen in the gaseous emanations of the 

 fissures of volcanoes, and the streams of lava 

 that have not yet cooled, many chemical con- 

 siderations, in a word, rise up in opposition to 

 the earlier conjectures of Davy and Ampere(^^*). 

 Were hydrogen evolved during eruptions of 

 lava, how enormous must its quantity prove in 

 cases where, from the low level of the point 

 whence the eruption flows, the outpouring mass 

 spreads over many square miles of surface, and, 

 dammed up in its course, acquires a thickness 

 of several hundred feet ; as happened in the 



