Mr Poulett Scrope's Considerations on Volcanos. 339 



3. The external pressure on the surface of the intumescent lava. 



Of these elements, the last is the most subject to variation, particularly 

 from changes, 1. In the dimensions of the vent ; and, 2d, In the quan- 

 tity and weight of matter pressing on the surface of the lava within the 

 vent. 1. The violent rise and explosive escape of the elastic fluids must 

 at first break up and enlarge the fissure, and consequently, the energy 

 of the eruption will progressively increase from its commencement; 

 but, 2d, the weight and consolidation of the lava protruded from 

 the orifice, and above all, the immense accumulation of fragmentary ejec- 

 tions within and around the vent, must, before long, give the predomi- 

 nance (unless under extraordinary circumstances) to the force of repres- 

 sion ; the crisis of the eruption is past, its violence diminishes progres- 

 sively, and it is at length wholly checked. Hence, a general law is de- 

 duced, that the developement of volcanic action universally tends to its 

 own extinction by augmenting the opposite force of repression. 



The author then considers the condition at this time of the dilated mass 

 of lava below, or the focus. Its temperature has been suddenly lowered 

 below that of the surrounding crystalline mass — it therefore abstracts ca- 

 loric from thence. If this accession of caloric keeps pace exactly with the 

 increase of the repressive force, the eruption is permanent. If not, the 

 continual increase of the expansive force in the lower parts of the crystal- 

 line bed, resolidifies the upper parts, and seals up the vent. 



But the expansive force of the focus continues to increase, and, perhaps 

 at length overcomes the resistances opposed to it. If it break out repeat- 

 edly in the same direction, it produces an habitual volcano, and finally a 

 volcanic mountain. 



If the repressive force prevail till the focus is equalized in temperature 

 to the stratum in which it lies, it shares in the general expansive force of 

 that stratum. This is continually increasing, and must at length find a 

 vent, generally on the same spot as before, and hence the frequency of ha- 

 bitual volcanos. If on a fresh point, probably on the continuation of the 

 original fissure, the rocks having been shattered along that line by the 

 earlier shocks ; hence the linear trains of volcanic vents so often noticed. 

 The distance of the new from the former vent, must depend on local cir- 

 cumstances in the structure, tenacity, and other elements of resistance in 

 the overlying rocks. In this manner, the draught of caloric, passing from 

 the great reservoir below to the exterior of the globe, is shifted from 

 one vent to another ; the focus of each active volcano abstracting ca- 

 loric from its inclosing walls ; neighbouring vents will also more or less 

 retard the activity of each other ; and the extreme energy of one may 

 cause the absolute extinction of the other. Other more tranquil modes of 

 escape for subterranean caloric are found by the author in thermal springs, 

 which result from the condensation of aqueous vapour percolating through 

 minor crevices from the subterranean heated lava-rock. So long as by 

 these, or other modes, the caloric passes off in the ratio in which it is re- 

 ceived from below, the general expansive force remains invariable. If 

 not, this force increases, and must at length prevail over the united I. 



