VOL 



[457] 



VOL 



cations of mineral volcanic pro- 

 ducts have been proposed, among 

 which the division into Trachytic 

 and Basaltic seems to be that most 

 commonly adopted ; trachyte being 

 considered as essentially composed 

 of felspar, and containing crystals 

 of glassy felspar; while basalt is 

 supposed to be essentially composed 

 of felspar, augite, and titaniferous 

 iron. Lavas, however, present such 

 various mixtures of different mine- 

 rals, that exact classifications of 

 them would appear exceedingly 

 difficult." De la Beche. The 

 principal gases hitherto detected 

 consist, according to Dr. Daubeny, 

 of muriatic acid gas, sulphur com- 

 bined with oxygen or hydrogen, 

 carbonic acid gas, and nitrogen. 

 The sublimations of Vesuvius are, 

 according to Sir H. Davy, chloride 

 of sodium, chloride of iron, sulphate 

 of soda, muriate and sulphate of 

 potash, and a small quantity of 

 oxide of copper. The principal 

 metallic substances in volcanic rocks 

 are iron and titanium ; but ores of 

 antimony, copper, and manganese, 

 have sometimes been found in the 

 craters of volcanoes. Tellurium, 

 gold, and mercury are also said 

 to occur in some volcanic rocks. 

 The island of Ischia, which is 

 entirely volcanic, contains a mine 

 of gold. 



VOLCA'NIC BOCKS. These are not 

 deemed synonymous with Plutonic 

 rocks. While the Plutonic rocks 

 are supposed to have been formed 

 by igneous action at great depths, 

 the Volcanic have risen up from 

 below, and have cooled from a 

 melted state upon or near the sur- 

 face. Volcanic rocks belong to 

 every period. 



VO'LCANITE. Another name for augite. 



VOLCA'NO. (from Vulcan, the god of 

 fire.) An opening in the earth's 

 surface made by internal fire. Vol- 

 canoes exist in all quarters of the 

 globe, and, according to Jameson, 



about one hundred and ninety-three 

 active volcanoes have been observed ; 

 of which 13 belong to Europe and 

 its islands, 66 to Asia and its 

 islands, 8 to the islands of Africa, 

 and 106 to America and its is- 

 lands. Volcanoes have been long 

 considered in the light of safety 

 valves, and this was the opinion 

 entertained of them by Strabo, and 

 it can scarcely be doubted that the 

 tranquillity of the incandescent fluid 

 mass, composing the earth's centre, 

 is owing to these numerous vents 

 for the passage of steam, &c. Al- 

 though volcanoes generally exist 

 in the neighbourhood of the sea, 

 yet this is not invariably the case, 

 as was once supposed. In central 



Asia there is a volcanic region with 

 an area of 2500 square geographical 

 miles, at a distance of upwards of 

 300 leagues from the ocean. 



VO'LVA. (volva, Lat.) In botany, a 

 species of calyx. A term applied 

 to a membranous wrapper or cover- 

 ing of the fungus tribe, which 

 conceals their parts of fructifica- 

 tion: in due time it bursts, and 

 forms a ring upon the stalk. 



VOLVA'RIA. A genus of cylindrical 

 convoluted shells known only in a 

 fossil state. The spire is not ex- 

 truded; aperture narrow, extending 

 the length of the shell : the colu- 

 mella plicated at its base: outer 

 lip dentated. Volvaria approaches 

 very near to Bulla cylindrica. 



VOLU'TA. A genus of simple marine 

 univalves, found in sands and mud 

 at depths varying from seven to 

 fourteen fathoms. Two species of 

 voluta, V. luctator and V. bicorona 

 are described as occurring in the 

 London clay ; one species V. lucta- 

 tor in the Bognor sandstone ; and 

 one species, V. ambigua, in the 

 chalk marl. 



VOLITION. A spiral wreath or turn. 

 The wreaths or turnings of the 

 shells of univalves are termed vo- 

 lutions. 



