GEOGNOSY AND GEOLOGY. 209 
many countries, and even in Germany in the Rhine region. The outbreaks 
of very many belong to ante-historical periods ; no doubt, however, can 
arise, as to their true character, as most still exhibit traces of former activity 
in the shape of lava currents, &c. These lava streams, in many cases, are 
exceedingly altered by the action of weathering and of water; the craters, 
also, may either be in good preservation, or marked by the same atmospheric 
influences. 
Pseudo-voleanic must be distinguished from truly volcanic phenomena. 
The two often exhibit great similarity, but are effects of different causes. 
Among these pseudo-volcanic exhibitions belong those terrestrial ignitions 
effected by the combustion of oxydizable substances in coal beds, such as 
are met with in various portions of England, Germany, and North 
America. 
_ At Zwickau in Saxony, the ground is heated to such a degree, that all 
the conditions necessary to the existence of a hot-house are answered, by 
simply erecting an edifice on the heated portion. In these hot-houses, 
without additional artificial warmth, such tropical fruits as pine-apples, &c., 
may readily be reared. At Dudley, in England, the subterranean fire may 
be seen through fissures in the rocks in dark nights: smoke and vapors 
habitually rise out of these fissures, and are visible at all times, especially 
in wet weather. PJ. 52, fig. 4, represents the burning mountain near 
Duttweiler. A conspicuous illustration of the same phenomenon is exhibited 
in Schuylkill county, Pennsylvania, where the rubbish from an extensive 
coal mine became ignited, and finally the whole bed. The result of such 
combustion is naturally to effect transformations in the neighboring rocks, 
bearing a considerable resemblance to those produced by regular abnormal 
masses while incandescent. At Epterode, at the foot of the Meissner, in 
Hesse, there is a hill originally consisting of tertiary clay, which has been 
changed by the combustion of a coal bed into a slag-like compact rock, the 
so called porcelain jasper. 
It was upon these subterranean combustions that Werner based his 
volcanic theory, which, however, meets with no support in the present state 
of science. 
Volcanoes are found in all parts of the world, and are confined to no 
particular level. They sometimes crown the ridges of widely extended 
mountain chains, as on the South American Andes; sometimes they rise up 
in mountainous or hilly regions and planes, and even from the bottom of the 
sea. The carefully conducted investigations of recent observers have 
shown that they are almost always in the neighborhood of the sea. Thus 
in Chili, in Peru, and in Mexico, they extend along the coast at no great 
distance ; in Europe, they lie along the Mediterranean Sea and the Atlantic 
Ocean. Most generally they occur on islands, many of which, unques- 
tionably, owe their very origin to the elevation of the incumbent volcano. 
There are volcanoes, however, which lie entirely within the main land, 
among which may be mentioned the extinct cones in China, France, and 
en the Rhine. Still, we may readily reconcile this fact with the general 
law of the contiguity of volcanoes to the sea, by reflecting that the sea 
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