Volcanos. 117 



escape of the elastic fluids must at first break up and enlarge the 

 fissure, and consequently, the energy of the eruption will progres- 

 sively 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 ejections within 

 and around the vent, must, before long, give the predominance, 

 (unless under extraordinary circumstances,) to the force of re- 

 pression ; the crisis of the eruption is past, its violence diminishes 

 progressively, and it is at length wholly checked. Hence, a 

 general law is deduced, 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 caloric from thence. If this acces- 

 sion of caloric keeps pace exactly with the increase of the re- 

 pressive 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 resistance opposed to it. 

 If ft break out repeatedly 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 gen- 

 eral expansive force of that stratum. This is continually increas- 

 ing, and must at length find a vent, generally on the same spot as 

 before, and hence the frequency of habitual 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 no- 

 ticed. The distance of the new from the former vent, must de^ 

 pend on local circumstances 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 caloric from its in- 

 closing 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 

 vapor percolating through minor crevices from the subterranean 

 heated lava-rock. So long as by these, or other modes, the ca- 

 loric passes off in the ratio in which it is received from below, 



