Ch. XV.] OF THE PRIMARY ROCKS. 367 



that they have various dimensions ; that they cross each other 

 in certain determinate directions ; that they are composed 

 either of solid earthy minerals, of ores, or of loose matter, 

 such as clay or friable quartz (which are probably only the 

 first in various stages of decomposition), some or all of which 

 are frequently united in different proportions. 



Having considered the nature of the intersection of veins 

 as an indication of their having been formed at different 

 times, we will next discuss the apparent movements by 

 which these intersections are supposed to have been accom- 

 panied. 



In the most simple form of such an occurrence, the prevail- 

 ing opinion on this subject is very plausible, and seems to be 

 confirmed by the fact, that in the mines, portions of the in- 

 tersected vein have been observed to follow the side of the 

 traversing one, so as to point out, as it were, the direction and 

 the nature of the displacement : but, when we contemplate the 

 curvatures, both in length and depth, of the supposed fissure, 

 (now occupied by the substance of the cross vein,) the cor- 

 responding convexities and concavities of its opposite sides 

 appear to be perfectly incompatible with such movements ; for 

 the fissure would have been obliterated thereby, in many 

 parts where the vein now possesses its full dimensions. Now, 

 as regards the indications of ore along the cross vein, inter- 

 mediate between the discontinuous veins, we do not, on the 

 supposition of both being contemporaneous, think it an un- 

 warrantable conjecture, that the power of cohesion, electro- 

 magnetism, or whatever force governed their arrangement, 

 should have left at the points of interference some traces of 

 the joint action of the intersecting currents. However, be this 

 as it may, how can we surmount the difficulty, that one vein 

 crossing another, without any movement in one place, in 

 others at different depths, produces heaves of various extent, 

 and vice versa, as in Dolcoath mine ? 



When more complicated instances of intersection are ex- 

 amined, the imperfection of the theory is still more evident : 



