1823.] M, Humholdt on VoUdnoei. 133 



closed again. The veins of basalt, dolerite, and porphyry, 

 which, in different parts of the world, pass through every forma- 

 tion; and those of syenite, augite-porphyry, and amygdaloid, 

 which are characteristic of the newest strata of the transition 

 formation, and of the oldest rocks of the secondary strata, have 

 probably been formed in a similar manner. In the first age of 

 our planet, the yet liquid substances penetrated through the 

 crust of the earth, which was every where intersected by 

 fissures, and assumed the form of granular rocks, either in veins, 

 or spreading over and expanding themselves in strata. The rocks 

 strictly volcanic which the primitive ages have aiforded us, have 

 not flowed in currents like the lava of our insulated conical hills ; 

 the same mixture of augite, titaniferous iron, glassy felspar, and 

 hornblende, may have existed at different periods, but at one 

 time it may have approached nearer to basalt, and at others to 

 trachyte ; the chemical substances may have combined in a 

 crystalline form, in distinct proportions, as we are taught by 

 M. Mitscherlich's new and important labours, and by the ana- 

 logy of artificial products of fire : we find that substances 

 similarly formed have arrived at the surface of the earth 

 in very different ways ; they have either been merely raised, or 

 protruded by temporary fissures through the older strata; that 

 is to say, through the already oxidized surface of the earth ; or 

 they have flowed, as currents of lava, from conical hills with a 

 permanent crater. By confounding such different phoonomena 

 together, the geognosy of volcanoes is carried back to that dark- 

 ness from which a great number of comparative observations are 

 beginning to extricate it. 



The question has often been asked. What is it that burns in 

 volcanoes ? What was it that excited the heat by which earths 

 and metals were melted? Modern chemistry answers, that 

 the substances w^hich melt are the metals of the earths and 

 alkalies. The solid crust of the earth, already oxidized, se- 

 parated the surrounding air with its oxygen, from the com- 

 bustible unoxidized substances of the interior of our planet. 

 The observations which have been made in mines and caves 

 in every zone, and which, in conjunction with M. Arago, I have 

 collected in a particular paper, demonstrate that the heat of the 

 mass of the earth is yet much greater than the mean temperature 

 of the atmosphere at the same place. Such a remarkable and 

 almost generally proved fact, is closely connected with those 

 which are proved by volcanic phsenomena. Laplace has even 

 gone so far as to endeavour to calculate the depth at which the 

 body of the earth may be considered to be a melted mass. 

 Whatever doubts may be entertained, notwithstanding the vene- 

 ration due to so great a name, with respect to the numerical 

 certainty of such a calculation, thus much remains probable ; 

 that all volcanic phsenoraena originate in a very simple cause, in 



