326 D. Forbes — On Volcanos. 



earliest ages, — taking for granted the generally received opinion that 

 the earth was originally a sphere of molten matter, the exterior of which 

 cooled and solidified to form a crust over the still liquid matter in 

 its interior, — after stating that the contraction of this crust, when 

 cooling, would cause cracks or fissures in it, through which some of 

 the stiil molten matter below would be forced up, or, in other words, 

 form the first lava streams or volcanic outbreaks, I remarked : '•' The 

 sides of these cracks or fissures would also be frequently more or 

 less dislocated, and so form lines of faults, interrupting the previously 

 regular contours of the surface, and forming the first elevations or 

 mountains on the globe, which, by giving direction to the action of 

 the water in the ocean and rivers, would in great measure determine 

 the main features of the physical geography of this epoch," — adding, 

 that such primeval causes, by regulating the demarcation and depo- 

 sition of sedimentary matter, must directly or indirectly continue to 

 influence the main features of external surface configuration, even 

 down to the present day. 



There are also good proofs that the great lines of disruption or 

 weakness seen in the earth's crust at present, have, at least in many 

 instances, also been lines of weakness in other and probably in even 

 the very oldest geological epochs. No better example of this can be 

 seen than in that greatest of all mountain ranges, the Andes, where, 

 commencing from the oldest period of their elevation, we find a 

 series of eruptive rocks breaking, one after the other, through the 

 sedimentary strata which form their flanks, as follows : — first, the 

 auriferous granites, probably at the end of the Devonian period; 

 second, the characteristic porphyrites of the Oolitic age ; third, the 

 auriferous diorites, disturbing the Cretaceous formation ; then, fourth, 

 the Miocene basalts ; and, fifth or lastly, the lavas from the present 

 volcanic fissures which occur at intervals along the whole range, 

 from Tierra del Fuego in the south, all through the Cordilleras of 

 South and Central America, up to the Eocky Mountains. 



If now, when studying the relative energy displayed by volcanic 

 forces in the older geological periods, we bear in mind that we still 

 have volcanos whose craters, several miles in diameter, send forth at 

 times streams of molten stone 40 miles and more in length, or 

 showers of ashes which bury the surface of the ground to a depth of 

 400 feet below them, and, furthermore, see volcanic mountains and 

 islands literally rising up before our eyes to an elevation of even 

 thousands of feet, in what, geologically speaking, is but a second of 

 time, it does not to me seem at all necessary to assume that such 

 internal or cataclysmic forces were so much more energetic in any 

 other period than at present. 



In the infancy of the science, the cradle of geology was in the 

 last century rent asunder, figuratively speaking, by the schools of 

 Neptune and Pluto, insisting each, in its turn, on the all-powerful 

 action of water or fire in the formation and subsequent changes 

 which have taken place in the earth ; and it is to be feared that even 

 now much of this old leaven remains, for there seems to be, as it 

 were, an innate tendency in geologists to attach themselves to some 



