Cs. XXXVIII] SWELLING OUT OF VEINS. 625 



A large proportion of metalliferous veins have their opposite walls 

 nearly parallel, and sometimes over a wide extent of country. There 

 is a fine example of this in the celebrated vein of Andreasburg in the 

 Hartz, which has been worked for a depth of 500 yards perpendicularly, 

 and 200 horizontally, retaining almost every where a width of 3 feet. 

 But many lodes in Cornwall and elsewhere are extremely variable in 

 size, being one or two inches in one part, and then 8 or 10 feet in an- 

 other, at the distance of a few fathoms, and then again narrowing as 

 before. Such alternate swelling and contraction is so often characteristic 

 as to require explanation. The walls of fissures in general, observes Sir 

 H. De la Beche, are rarely perfect planes throughout their entire course, 

 nor could we well expect them to be so, since they commonly pass 

 through rocks of unequal hardness and different mineral composition. 

 If, therefore, the opposite sides of such irregular fissures slide upon each 

 other, that is to say, if there be a fault, as in the case of so many mineral 

 veins, the parallelism of the opposite walls is at once entirely destroyed, 

 as will be readily seen by studying the annexed diagrams. 



Fig. 714. 



Let a b, fig. 714, be a line of fracture traversing a rock, and let a b, 

 fig. 715, represent the same line. Now, if we cut a piece of paper re- 

 presenting this line, and then move the lower portion of this cut paper 

 sideways from a to a', taking care that the two pieces of paper still touch 

 each other at the points 1, 2, 3, 4, 5, we obtain an irregular aperture at 

 c, and insolated cavities at d d d, and when we compare such figures 

 with nature we find that, with certain modifications, they represent the 

 interior of faults and mineral veins. If, instead of sliding the cut paper 

 to the right hand, we move the lower part towards the left, about the 

 same distance that it was previously slid to the right, we obtain consid- 

 erable variation. in the cavities so produced, two long irregular open spa- 

 ces,//, fig. 716, being then formed. This will serve to show to what slight 

 circumstances considerable variations in the character of the openings 

 between unevenly fractured surfaces may be due, such surfaces being 

 moved upon each other, so as to have numerous points of contact. 



Most lodes are perpendicular to the horizon, or nearly so ; but some 

 of them have a considerable inclination or " hade," as it is termed, the 

 angles of dip varying from 15° to 45°. The course of a vein is frequent- 

 ly very straight ; but if tortuous, it is found to be choked up with clay, 



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