218 j. logan lobley^ f.g.s., f.r.g.s., on volcanic action 



Geological and Geographical Conditions and Eesults 

 OF Volcanic Action. 



For a due estimation of the sufficiency of any hypotliesis of 

 volcanic action it is, however, not enough to consider only the 

 phenomena of eruptions. The larger facts, both geological and 

 geographical, consequent upon, or incidental to, volcanic action^ 

 not only in the present epoch, but during past periods of the 

 earth's history, must be taken into consideration, and their 

 relative importance carefully valued. 



These are so numerous and varied that a volume would be 

 required to do justice to their interesting and important 

 characters. All that can be done here is to oive a brief 



o 



summary, and with this I must now content myself. 



The outputs of volcanic eruptions relatively to the bulk of 

 the globe are individually infinitesimal, and in their aggregate 

 form only a small part of even the land surface of the earth. 



There is no general constant flexibility of the earth's surface,, 

 each subsidence or upheaval Ijeing local and of limited duration, 

 while astronomical calculations and the general stability of land 

 and sea during long periods prove great rigidity of the exterior 

 of the globe as a whole, and consequently a great thickness of 

 solid rocky substructure. 



The inorganic Paheozoic conditions of the earth's surface were 

 generally similar to those of Xeozoic times, as shown by similar 

 organisms, ripple-marks, worm-burrowings, rain-pittings, etc.,. 

 and Pafi-eozoic volcanic action does not appear to have been 

 greater than ISTeozoic, while the highest mountain ranges have 

 received a large amount of their present elevation since the close 

 of the secondary period. 



The specific gravity of the globe is only 5'5, although that of 

 surface rocks is over 2"5, while the pressure at the centre is 

 calculated bv Walterhausen at 2,498,600 atmospheres, and by 

 Laplace at 3";000,660. 



Heat increases with depth below the surface at a rate that if 

 continued would give rock-fusion, under atmospheric pressure 

 only, at from 25 to oO miles, and at half the distance to the 

 centre a temperature equal to that of tlie sun, an impossible 

 heat, and consequently there is not a continued uniform increase 

 of heat. 



Eock-fusion resulting from relief of vertical pressure in 

 subterranean regions would not, with, an open vent, be limited 

 in lateral extension, and surface ruptures would be produced 

 with lava outputs on a scale far transcending any catastrophes. 



