MODIFIED by COMPRESSION. 161 



preflure would in all cafes, efpecially where the depth was confi- 

 derable, far furpafs what it would have been under an equal 

 depth of water. Where the depth of the ftream, below its 

 point of delivery, amounted, then, to 1708 feet, the prefiiire, if 

 the heat was not of exceffive intenfity, would be more than fuf- 

 flcient to conftrain the carbonic acid, and our limeftone would 

 fufFer no calcination, but would enter into fufion ; and if the 

 eruption ceafed at that moment, would cryftallize in cooling 

 along with the lava, and become a nodule of calcareous fpar. 

 The mafs of lava, containing this nodule, would then conftitute 

 a real whinftone, and would belong to the kind called amygda- 

 loid. In greater depths ftill, the preflure would be propor- 

 tionally increafed, till fulphur, and even water, might be con- 

 {trained ; and the carbonate of lime would continue undecom- 

 pofed in the higheft heats. 



If, while the lava was in a liquid ftate, during the eruption 

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

 country below the volcano, was met by our ftream (at d), it is 

 obvious that the lava would flow into the aperture with great 

 rapidity, and fill it to the minuteft extremity, there being no 

 air to impede the progrefs of the liquid. In this manner, a 

 ftream of lava might be led from below to approach the bot- 

 torn of the fea (//), and to come in contact with a bed of 

 loofe fhells (gg), lying on that bottom, but covered with 

 beds of clay, interftratified, as ufually occurs, with beds of 

 fand, and other beds of fhells. The firft effect of heat would be 

 to drive off the moifture of the loweft fhell-bed, in a ftate of va- 

 pour, which, rifing till it got beyond the reach of the heat, 

 would be condenfed into water, producing a flight motion of 

 ebullition, like that of a veflel of water, w r hen it begins to 

 boil, and when it is faid to fimmer. The beds of clay and fand 

 might thus undergo fome heaving and partial derangement, but 

 would ftill poflefs the power of flopping, or of very much im- 



Vol. VI.— P. I. X peding, 



