200 Effects of Heat modified l>y Compression. 



cnmplish their entire fusion. The mountain of Saleve, near 

 Geneva, is 500 French fathoms, or nearly 3250 English feet, 

 in height, from its base to its summit. Its mass consists 

 of beds, lying nearly horizontal, of limestone filled with 

 shells. Independently, then, of the tenacity of the mass, 

 and taking into account its mere weight, the lowest bed of 

 this mountain must, at this moment, sustain a pressure of 

 3250 feet of limestone, the specific gravity of which is about 

 Q-65. This pressure, therefore, is equal to that of S612 feet of 

 water, being nearly a mile and a half of sea, which is much 

 more than adequate, as we have shown, to accomplish the 

 entire fusion of the carbonate on the application of proper 

 heat. Now, were an emanation from a volcano to rise up 

 under Saleve, and to penetrate upwards to its base, and 

 stop there, the limestone to which the lava approached 

 would inevitably be softened without being calcined, and, 

 as the heat retired, would crystallize into a saline marble. 



Some other circumstances relating to this subject are very 

 deserving of notice, and enable us still further to compare^ 

 the antient and modern operations of fire. 



It appears, at first sight, that a lava, having once pene- 

 trated the side of a mountain, all subsequent lavas should 

 continue, as water would infallibly do, to flow through the 

 game aperture. But there is a material difference in the 

 two cases. As soon as the lava has ceased to flow, and the 

 heat has begun to abate, the crevice through which the lava 

 •had been passing remains filled with a substance, which soon 

 agglutinates into a mass far harder and firmer than the moun- 

 tain itself. This mass, lying in a crooked bed, and being 

 firmly welded to the sides of the crevice, must oppose a 

 most powerful resistance to any stream tending to pursue the 

 same course. The injury done to the mountain by the 

 formation of the rent, will thus be much more than re- 

 paired, and in a subsequent eruption the lava must force its 

 way through another part of the mountain or through some 

 part of the adjoining country. The action of heat from below 

 seems in most cases to have kept a channel open through 

 the axis of the mountain, as appears by the smoke and flame 

 which is habitually discharged at the summit during inter- 

 vals 



