166 VOLCANIC CONES AND LAVA CURRENTS. 



different causes. We know, in fact, that during earthquakes, rivers as well 

 as lakes suddenly disappear under ground, sometimes temporarily and 

 sometimes continuously ; it is conceived that the water flows through internal 

 cracks, similar to those produced on the surface, which form canals for its 

 passage. The phenomenon is sometimes coincident with tho appearance 

 of some abundant spring in a more or less distant place ; but it often hap- 

 pens also that the water nowhere re-appears, and we must conclude that it 

 runs directly into the sea. All these circumstances explain the disappear, 

 ance of certain rivers, which are swallowed by the earth after a superficial 

 course of more or less extent, as well as the sudden appearance of springs 

 gushing from the side of a rock. They point to the existence of subterra- 

 neous canals, and lead us to think that, dried up by a more or less consider, 

 able upheaval, these canals may have formed the now empty caverns found 

 at all heights, as well as those, the bottom of which are still occupied by a 

 stream of water fed from lakes or rivers on the surface. 



Still, if the real origin of most of these subterraneous cavities be not 

 doubtful, it must be admitted that subsequently important changes took place 

 in the general form and condition of their parietes ; the rounded form, wear 

 and polish of surfaces, grooves, different excoriations, and in all positions, 

 even on the upper part of the vault, an erosive action of which water alone 

 is incapable. It has been thought this liquid might have been charged with 

 carbonic acid gas, which is frequently disengaged from the earth through 

 fissures formed in it, particularly at the time of earthquakes, and that the 

 subsequent effects were owing to its dissolving power. 



LESSON IX. 



EXPLANATION OF VARIOUS PHENOMENA CONTINUED. Deposits 

 attributable to Volcanic Action Lava Basa'lt fiction of 

 Basalt on Adjacent Rocks Dolomisation Giant's Causeway 

 Tra'chytic Formation Trap Rocks Porphyry Granitic 

 Rocks 'injection of Granite Metalliferous Veins Met a- 

 mo'rphism Effects of Erosion. 



1. Volcanic cones and lava currents. When we find conical 

 bills isolated, or arranged several together on a line, and covered 

 with scoriae, sometimes having crate'riform cavities at 

 the summit, surrounded by rapilli, we may be certain 

 they are volcanic cones, however ignorant we may be 

 of" the epoch of their activity. If on mountain sides, 

 whatever may he their nature, we see long, straight 

 masses., terminated^below in a club, hollow in the mid- 

 dle, and thinning out above in a pellicle of dislocated 

 scoriae (fig. 274), their origin cannot be doubtful, 

 although every other trace of volcanic action may 

 have disappeared. These long, straight masses are 

 lava currents. If we find these matters in pebbles, in 

 more or less extensive tables, compact below, porous, 

 tig. ~74. ce ji u } arj or scoriaceous above, with a nearly uniform 



1. By what features are extinct volcanoes recognised ? 



