208 GEOGNOSY AND GEOLOGY. 
veins, although they themselves never contain these. Where basalts occur, 
they are generally combined with basalt conglomerate, which is often of 
great thickness. It generally lies at the foot, or on the slopes of basaltic 
mountains, and where it is tufaceous, it appears stratified, sometimes even 
alternating with strata of brown coal and woody opal. Where the basalt 
stands in contact with stratified rocks, the latter are influenced in the most 
varied manner, in a manner entirely attributable to the elevated temperature. 
Sand is converted into quartz grit, limestone into marble;  silicious 
substances, as jasper, chalcedony, and hornstone, are forced into sandstone 
and lime, and partly melted together. Gypsum likewise is found in the 
vicinity of basalt. 
A magnetic polarity has been ascertained to exist in basalt as well as in 
granite, dependent, in all probability, upon the magnetic oxyde of iron. 
Basalt experiences a chemical decomposition, which is first indicated by 
a rusty coating to-the surface. The soil resulting from such decomposition 
is often very fertile, and calculated to the formation of swamps. 
Basalt is distributed in Greenland, Iceland, on the Faroes, in the Hebrides 
and in Ireland, also in Germany, France, Spain, Portugal, Italy, North 
Africa, America, on the South Sea Islands, and in the East Indies. In 
Germany it is seen on the Leine, on the Weser, in Hesse, in the Rhineland 
about Bonn and Coblentz, in Thuringia, in the Rhongebirge, in Lausatia, 
Bohemia, Héhgau, &c. 
3. Voucanic Rocks. 
Volcanoes, among the most conspicuous of all geological phenomena, stand 
in the same relation to the other abnormal masses as the top rocks do to the 
normal or stratified; they come immediately after the voleanoid masses, 
from which, however, they are essentially different, although in many cases 
it is quite difhcult to draw the line of distinction. 
By volcanoes or burning mountains are generally to be understood conical 
elevations, with an apical concavity, in communication, by a deep hole or 
throat, with the interior of the earth, through which liquid masses and solid 
rocks are ejected from time to time. The concavity or crater, and the 
descending funnel, are characteristic of the volcano, although these features 
may be masked by the crumbling and falling in of the sides. Pl. 44, fig. 
Y, is a section of a volcanic cone. Volcanoes may be divided into two 
classes, active and extinct, with an intermediate form, the Solfatara, where 
there is a continued emission of sulphurous matters. 
Active volcanoes have often long periods of rest or intermission, after 
which they again become active, and are so much the more devastating. 
An example of this is found in Vesuvius, whose eruptive history begins from 
the time of Pliny, and continues to the present time. Although there 
probably were eruptions anterior to the time of Pliny, yet of such we possess 
no record. 
Extinct volcanoes are much more numerous than active; they occur in 
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