140 ORIGIN AND DEVELOPMENT OF ORE DEPOSITS. 



quantity. The existence of " sandcourses," as at Great Beam 

 Mine* and East Wheal Kose, probably results from a similar 

 grinding motion, the clay having been carried away by the 

 waters to another part of the fissure. 



Intersecting fissures. Assuming that one of the mechanical 

 effects of the elevation of a "mountain system" is the produc- 

 tion of two sets of vertical or inclined fissures, one parallel to 

 the axis of elevation, the other at right angles to it, we have 

 now to consider the probable effect of a second system of 

 upheaval acting in or near the same region. Should the new 

 axis actually coincide with the old one, or be at right angles to 

 it, the tendency would be to re-open the old fissures, and perhaps 

 to form some new ones parallel in bearing but with different 

 underlie. Even if the new system should not be exactly 

 parallel or at right angles to the old, if for example there should 

 be a difference of 10° or so, many of the old fissures would still 

 be re-opened in part. But in many cases also we should have 

 new fissures formed in portions of the rock-mass which had before 

 escaped Assuring. Thus, in fig. 12, Plate X, a. a. and h.b. being 

 fissures of a primary system parallel to the axis AA., and dipping 

 in the direction of the arrow-heads, if a new axis of elevation 

 B.B. were subsequently formed, we might expect the new 

 fissures having similar dip to take the course of the black line 

 c. a. c. a. c. But if the old fissures were already very strongly 

 repaired, the new fissures would probably go directly in the 

 course/;/., and this would certainly be the case with new fissures 

 dipping in the opposite direction. In proportion, too, as the 

 angles between the new and the old system approached 45° in 

 that same proportion would there be a tendency for entirely new 

 fissures to form, while, if the angle became greater than 45°, 

 the tendency would be to re-open the corresponding "perpen- 

 dicular system."f 



To a certain extent all new fissures must tend to the 

 obscuration of old ones, especially when four or five " systems" 

 are superposed, as must frequently happen in a geologically 



* See The Hensbarrow Granite District, by J. H. Collins, Truro, 1878, p. 39. 



fThis subject of fissures made up of " elements " from various systems, 

 producing "turns," "warps," or "inflexions," is fully dealt with in Moissenet's 

 " Lodes of Cornwall," Chap. 1, Plates n and v. 



