GEOLOGY. S3 



PORPHYRY is a rock in which crystals of one or more 

 minerals are diffused through a compact or earthy base. 

 The crystals are generally of felspar, augite, or olivine ; 

 and the nature of the base gives the name to the por- 

 phyry. If the base be greenstone, claystone, or pitch- 

 stone, it is called greenstone porphyry, claystone porphyry, 

 and so forth. 



AMYGDALOID. This rock is so called from the word 

 amygdala, an almond, and comprises any rock which con- 

 tains round or almond-shape nodules of some mineral. 

 The porous cavities in this rock were most probably 

 formed through the expansion of vapour or gas when in 

 a state of fusion, which cavities, being filled up with a 

 solution of mineral substances afterwards became conso- 

 lidated. 



LAVA is a substance of a porous nature, which flows 

 from volcanoes in action, and is a composition of various 

 substances, as basalt, trachyte, &c. ; and PUMICE is of 

 a similar character, but much lighter, being fibrous and 

 spongy, through the expansion of gases by means of 

 heat. 



As the rocks just named are produced through Vol- 

 canic action, it may not be irrelevant to consider briefly 

 the cause and results of this most pOtent agency. The 

 presumed cause of VOLCANIC ACTION has beenvariously 

 conjectured by philosophers ; but a very probable cause 

 is often the oxidization of the basis of alkaline matter 

 through the access of water ; the water becoming de- 

 composed and liberating its hydrogen, which, by its 

 inflammable and elastic nature, might be sufficiently 

 powerful to rend asunder the solid earth, were it not 

 for these natural vents Volcanoes. 



Previous to a volcanic eruption, smoke appears from 

 the mountain, a rumbling of the earth is heard, earth- 

 quakes are felt (earthquakes being produced from the 

 same cause as Volcanic action), and presently an explo- 

 sion takes place, with an ejection of ashes and sand, 

 followed by a stream of melted lava. This action is not 

 entirely confined to mountains, but will sometimes take 

 place in the sea, which, by upheaving the superincum- 

 bent mass, sometimes forms islands. Nearly half of the 

 islands are of volcanic origin, as the Azores, the Canary 

 Islands, and others. 



