318 On the Paragenetlc Relations of Minerals. 



secretion. There are, indeed, circumstances which render 

 it probable that it has been introduced into the fissures from 

 below. And again there is no reason for assuming that tin 

 is not one of those elements which may have remained at a 

 considerable depth below the surface during the formation of 

 the earlier rocks. 



(C.) Lodes formed by Eruption. — Three different modes 

 of eruption must be admitted in any general theory of 

 these lodes : 1. The eruption of melted or at least pasty 

 masses ; 2. The eruption of solutions ; 3. Sublimation. Lodes 

 which have originated in the first of these modes do not 

 present the peculiar banded structure observed in most 

 others. Springs upon lodes are by no means uncommon, 

 and most of the true mineral springs of any particular dis- 

 trict are so situated, that their origin from a lode of some 

 kind is very probable. The formation of lodes by sublima- 

 tion may sometimes be observed at the present day on vol- 

 canic mountains. Thus, for instance, during an eruption of 

 Vesuvius in 1817, a fissure of more than three feet diameter 

 was filled within the space of ten days with specular iron ore, 

 deposited from the vapour of chloride of iron evolved. The 

 lodes of red hematite in the Upper Erzgebirge may have ori- 

 ginated in a similar manner, although more slowly. 



Manganese lodes are perhaps likewise deposits from super- 

 fluoride or chloride of manganese. Silica and the few sili- 

 cates may have been introduced by aqueous vapour. Bischof 

 is of opinion that metallic silver has originated from silver 

 glance by the abstraction of sulphur by steam. 



The principal grounds for the ascension theory are, — 1. 

 The minerals of lodes are chiefly such as in a chemical point 

 of view can only have been formed in the wet way ; but, to 

 judge from their constituents, are neither products of surface- 

 water or of the extraction of the adjoining rocks. 2. Certain 

 minerals are not unfrequently found in druses of the lodes 

 covering or implanted upon the underneath surfaces of crys- 

 tals. Thus, for example, at Lobenstein the rhombohedrons 

 of spathic iron have a double covering ; that on the under 

 surfaces being clear acicular quartz ; that on the upper sur- 

 faces clay, and these latter, when washed, have a less brilliant 



