74 



CLASSES OF ROCKS. 



ance, and lies many thousand feet lower, than 

 inland and more lofty countries. 



The volcanoes that are active at the present 

 time, and that communicate permanently by 

 craters vv^ith the interior of the earth and the 

 atmosphere, became open at so late an epoch, 

 that the superior cretaceous deposits, and the 

 whole of the tertiary formations, were already 

 in existence when they arose. This is pro- 

 claimed by the trachytes, and also by the basalts, 

 which frequently form the walls of the up- 

 heavement craters. Melaphyres extend to the 

 middle tertiary strata ; but have already begun 

 to show themselves under the Jura formations, 

 when they appear breaking through the varie- 

 gated sandstone(2''8). The active volcanoes of 

 the present time, communicating with the air 

 by craters, must not be conTounded with those 

 older eruptions of granite, quartzose porphyry, 

 and euphotide, through open, but speedily-clo- 

 sed fissures (forming veins), which occur in the 

 old transition strata. 



The extinction of volcanic activity is either 

 partial only, so that the subterranean fire finds 

 another vent in the same mountain chain ; or 

 it is total, as in Auvergne ; later examples are 

 supplied, in perfectly historical times, by the 

 volcano Mosychlos(=^°^), on the island dedicated 

 to Hephoestos, whose " upward flickering fiery 

 glow" was known to Sophocles, and by the 

 volcano of Medina, which, according to Burck- 

 hardt, threw out a stream of lava on the 2d 

 of November, 1276. Each stage of the volcan- 

 ic activity, from its first excitement to its ex- 

 tinction, is characterized by peculiar products : 

 first, by fiery scoriae, by trachytic, pyroxenic, 

 and vitreous lavas in streams, by scoriae and 

 tuff ashes, accompanied by the evolution of 

 large quantities of generally pure watery va- 

 pour ; at a later period as solfataras, when 

 there is an evolution of watery vapour mixed 

 with sulphuretted hydrogen and carbonic acid 

 gases ; lastly, when all has cooled, by exhala- 

 tions of carbonic acid gas alone. Whether that 

 singular class of burning mountains which dis- 

 charge no lava, but dreadful devastating streams 

 of hot waterC^'"), loaded with burning sulphur, 

 and rocks ground down to powder — suo.h, for 

 instance, as Galunggung, in the island of Java 

 — present us with what may be called a normal 

 condition, or only a certain transitory modifica- 

 tion of the volcanic process, will remain a ques- 

 tion undecided, until they have been visited by 

 geologists possessed at the same time of a 

 knowledge of modern chemistry. 



Such is the very general view of volcanoes, 

 so important an element in the life of the earth, 

 which I have here endeavoured to throw to- 

 gether. It is based, in part, upon my own ob- 

 servations ; in the generality and comprehen- 

 siveness of its outlines, however, upon the la- 

 bours of my friend of many years, Leopold von 

 Buch, the greatest geologist of our age, who 

 was the first to recognize the intimate connec- 

 tion of volcanic phenomena, and their mutual 

 interdependence in regard to their actions dnd 

 their relations in space. 



The reaction of the interior of a planet upon 

 its outer crust and surface, as manifested in 

 the phenomena of volcanoes, was long consid- 

 ered as a mere isolated phenomenon, and pe- 



culiar only with reference to the destructive 

 agency of its dark and subterraneous forces ; it 

 is but very lately, and greatly to the advantage 

 of that geology which is founded on physical 

 analogies, that the volcanic forces have begun 

 to be regarded as formative of new species of 

 rocks, and as transformative of older mineral 

 masses. Here, indeed, is the point already al- 

 luded to, where a more deeply-grounded doc- 

 trine of volcanoes in a state of activity, and ei- 

 ther casting out fire or vapour, leads us, in our 

 general Picture of Nature, by a double way, 

 the one to the mineralogical portion of geog- 

 nosy, or the doctrine of the structure and suc- 

 cession of the strata composing the crust of the 

 earth ; the other to the form and fashion of the 

 continents and groups of islands raised above 

 the level of the sea, or the doctrine of the geo- 

 graphical forms and outlines of the several por- 

 tions of the earth. Enlarged views of such an 

 enchainment of phenomena is a consequence 

 of the philosophical direction which the serious 

 study of geognosy has now so generally taken. 

 Greater perfection of the sciences leads, as in 

 the political improvement of mankind, to con- 

 nection and agreement, where there had for- 

 merly been separation and distinction. 



When we class rocks or mineral masses not 

 according to differences in the form and ar- 

 rangement of their constituent particles, into 

 stratified and unstratified, schistose and massy, 

 normal and abnormal rocks, but look at the 

 phenomena of formation and transformation 

 which are still going forward under our eyes, 

 we discover a four-fold process of production 

 in connection with rocks : 1st. Eruptive rocks, 

 rocks thrown out from the interior of the earth, 

 in a liquefied, or softened and more or less te- 

 nacious state (volcanic and Plutonic rocks). 

 2d. Sedimentary rocks, rocks deposited from 

 fluids in which the particles had been either 

 dissolved or suspended, but from which they 

 were precipitated and deposited upon the sur- 

 face of the crust of the earth. The greater 

 number of the floetz and tertiary groups. 3d. 

 Metamorphic rocks, rocks altered in their inti- 

 mate structure and stratification, either through 

 the contact and vicinity of a Plutonic or vol- 

 canic (endogenous) (=*") ejected rock, or — and 

 this is more commonly the case — altered by the 

 penetration of the vaporiform sublimed mat- 

 ters(2i2), vv^hich accompany the escape of certain 

 molten ejected masses. 4th. Conglomerates 

 — coarse or fine-grained sandstones, breccias — 

 rocks made up of mechanically divided masses 

 of the three former species. 



These four-fold rock-formations, which still 

 go on at the present day, through the effusion 

 of volcanic masses in the shape of streams of 

 lava, through the influence of these masses 

 upon rocks consolidated at a former period, 

 through mechanical separation or chemical pre- 

 cipitation from liquids charged with carbonic 

 acid, finally, through the cementation of frag- 

 ments often of totally different kinds of rocks, 

 are phenomena and formative processes which 

 can, however, only be regarded as weak reflec- 

 tions of what went on under the higher inten- 

 sity of action in the life of the earth during the 

 chaotic state of the primitive world, and under 

 totally different conditions of pressure and high 

 temperature, not only of the wliQle crust of the 



