774 CONTRACTION OF VEINS. [Oh. XXXVEZ 



wise in the case of dikes, where each opening has been the receptacle 

 of one continuous and homogeneous mass of melted matter, the con- 

 solidation of which has taken place under considerable pressure. 

 Trappean dikes can rarely fail to strengthen the rocks at the points 

 where before they were weakest ; and if the upheaving force is again 

 exerted in the same direction, the crust of the earth will give way 

 anywhere rather than at the precise points where the first rents were 

 produced. 



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 perpen- 

 dicularly, and 200 horizontally, retaining almost everywhere a width 

 of 3 feet. But many lodes in Cornwall and elsewhere are extremely 

 variable in size, being 1 or 2 inches in one part, and then 8 or 10 feet 

 in another, at the distance of a few fathoms, and then again narrowing 

 as before. Such alternate swelling and contraction is so often charac- 

 teristic 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 irregu- 

 lar 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. 766. 



Let a b, fig. 766, be a line of fracture traversing a rock, and let a 6, 

 fig. 767, represent the same line. Now, if we cnt in two a piece of 

 paper representing 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 isolated cavities at d d d ; and when we 

 compare such figures with nature we find that, with certain modifica- 

 tions, 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 



