140 Electro-magnetic properties of the mines of CornwalL 



in a pure state, or to the proportion of them in combination. Silver 

 and mercury, for example, are combined with, comparatively, very 

 small quantities of sulphur ; and zinc, which seems to hold an oppo- 

 site place to silver in the electrical- scale, is also found in combination 

 with a much less proportion of sulphur than is contained in copper 

 pyrites, though the latter is one of the best mineral conductors of 

 electricity. There are many other analogous examples, which 

 prove that do conclusion can be drawn, a priori, from the nature or 

 chemical arrangements of minerals, as to their relative electrical 

 properties. 



Much time and attention have been bestowed by geologists on the 

 consideration of the origin and comparative ages of veins, and but 

 little on the purposes for which they are designed. 



The author thinks it will prove a vain attempt to reconcile a mul- 

 titude of facts observable in mines with any known natural causes. 



1st. The very oblique descent of a large proportion of the veins 

 into the earth, in some cases in very hard rock, and in others in 

 ground so soft, that it would immediately fall in, however small the 

 excavation, without being completely supported by timber. Were 

 it possible to conceive fissures to exist under such circumstances, it 

 is not reasonable to suppose that diey would not take the direction in 

 which the resistance would be least, that is, either the vertical, or the 

 line of the cleavage of the rocks. 



2d. Veins are often divided into branches, which unite again at a 

 considerable depth, including between them vast portions of rock, 

 perfectly insulated by the ore or vein stones from the general mass; 

 these, it is evident, could not have existed as fissures for a moment. 



3d. Veins are continually subject to changes in their horizontal di- 

 rection and underlie ; their size also often varies exceedingly, one 

 part being many times wider than another, without any reference to 

 their relative position or depth under the surface. 



4th. Although a poruon of their vein stones are usually quite dis- 

 tinct in their characters from the rocks they traverse, they are gen- 

 erally, in part, of the same nature, and vary with the containing rocks, 

 whether granite, elvan, killas, &c., and they are commonly too regu- 

 larly arranged in the veins, and are found inclosing insulated portions 

 of the ore, Sec. in their very substance, to admit of the idea of their 

 having been originally mere broken fragments of the inclosing rocks. 



At Dolcoath mine, there is an instance of one ore vein intersect- 

 ing another at different depths, and being itself intersected, and even 

 |:.hifted by the same vein al a grciiter depth. 



