Effects of Heat modified hj Compression. ] 97 



in consequence of the bubbles of carbonic acid, and other 

 substances, in a gaseous form, which, rising with the lava, 

 and through it, would greatly diminish the weight of the 

 column, and would render its pressure on any particular spot 

 extremely variable. With all these irregularities, however, 

 and interruptions, the pressure would in all cases, especially 

 where the depth was considerable, far surpass what it would 

 have been under an equal depth of water. Where the depth 

 of the stream, below its point of delivery, amounted, then, 

 to 1708 feet, the pressure, if the heat was not of excessive 

 intensity, would be more than sufficient to constrain the 

 carbonic acid, and our limestone would suffer no calcina- 

 tion, but would enter into fusion; and if the eruption ceased 

 at that moment, would crvstallize in cooling along with the 

 Java, and become a nodule of calcareous spar. The mass 

 of lava containing this nodule, would then constitute a real 

 whinstone, and would belong to the kind called amygda- 

 loid. In greater depths still, the pressure would be propor- 

 tionally increased, till sulphur, and even water, might be 

 constrained; and the carbonate of lime would continue un- 

 decomposed in the highest heats. 



If, while the lava was in a liquid state, during the erup- 

 tion or previous to it, a new rent (de, fig. 41.), formed in 

 the solid country below the volcano, was met by our stream 

 (at d), it is obvious that the lava would flow into the aper- 

 ture with great rapidity, and fill it to the minutest extremity, 

 there being no air to impede the progress of the liquid. In 

 this manner, a stream of lava might be led from below to 

 approach the bottom of the sea (ff), and to come in con- 

 tact with a bed of loose shells (gg) lying on that bottom, 

 but covered with beds of clay, inlerstratified, as usually oc- 

 curs, with beds of sand, and ^ther beds of shells. The first 

 effect of heat would be to drive off the moisture oi the lowest 

 shell- bed in a state of vapour, which, rising tdl it got be- 

 yond the reach of the heat, would be condensed into water, 

 producing a slight motion oi ebullition, Jike that of a vessel 

 of water, when it begins to boil, and when it is said to sim> 

 liiL-r. The beds ot cla. and sand might thus undergo some 

 heaving and partial derangement, but would still poss< ss he 

 N 3 power 



