132 GEOGNOSY AND GEOLOGY. 
( fig. 86), the ramifications being sometimes occupied by ore of different 
richness from that of the rest of the vein. It occurs not unfrequently, that 
the vein B, crossed by the vein A, is entirely cut off ( fig. 87). 
Veins sometimes are found which consist of the combination, at every 
possible angle, of innumerable threads, weaving together entire mountain 
masses. This is the interlaced vein in which tin ores generallv occur. 
16. Theory of Veins. 
The true character of mineral veins is still involved in great obscurity. 
There can be no doubt that veins are the filling in of splits, fissures, or 
cracks, and that the origin of many veins, especially those which occur in 
abnormal masses, can be satisfactorily explained. The answering of the 
question, as to how these cracks arose, and in what manner they have been 
filled up, presents difficulties so great, that a long time must probably 
elapse before they are entirely removed. Many of the theories suggested 
are untenable on their very face; and others, not quite so preposterous in 
themselves, require more satisfactory verification than they have vet 
received. 
The assumption that vein fissures owe their origin to volcanic actions, 
among which we enumerate earthquakes, and the elevation of plutonic, 
volcanic, and voleanoid masses, possesses a great show of probability, since 
entire systems of veins may be reduced to certain points of elevation. 
Mount Etna furnishes remarkable illustrations of this kind. The veins 
belonging to one period of eruption all run more or less radial towards the 
eruption cone of the principal crater; these veins thus belong to the 
principal crater. Now there are many systems of veins, in which each 
vein has reference to the same point, or stands in such similar connexion 
with certain points, as to compel the supposition that a principal crater 
must at one time have formed the centre of such a system. The 
phenomenon is not rare indeed of a volcano so choking up its old crater, 
as that the molten matter in its elevation has been compelled to throw up 
a new crater, in connexion with whose formation new systems of cracks 
make their appearance. This explains the fact that veins are of most 
frequent occurrence in those primary rocks which are most intimately in 
communication with abnormal masses; ore veins are also frequently found 
on the limit between normal and abnormal masses, and are known as 
contact veins. 
The history of the hypotheses adduced to account for the phenomena of 
veins is much the same with that of the suggestions with reference to the 
origin of terrestrial volcanoes. The most improbable and exaggerated 
hypotheses have been propounded in this respect, many of them pure 
fancies of the imagination, supported on imperfect observations, while others 
again possess a greater amount of plausibility, and even in some cases 
establish laws which yet cannot be considered as applicable to all cases. 
Thus we may, with propriety, assert that veims of calcareous spar in 
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