Ch. XXXYIIL] 



ENLARGEMENTS of veins. 



623 



minerals, arranged with the utmost regularity on each side of the cen- 

 tral layer. This layer was formed of two beds of calcareous spar, which 

 had evidently lined the opposite walls of a vertical cavity. The thirteen 

 beds followed each other in corresponding order, consisting of fluor-spar, 

 heavy spar, galena, &c. In these cases, the central mass has been last 

 formed, and the two plates which coat the outer walls of the rent on 

 each side are the oldest of all. If they consist of crystalline precipi- 

 tates, they may be explained by supposing the fissure to have remained 

 unaltered in its dimensions, while a series of changes occurred in the 

 nature of the solutions which rose up from below ; but such a mode of 

 deposition, in the case of many successive and parallel layers, appears to 

 be exceptional. 



If a veinstone consist of crystalline matter, the points of the crystals 

 are always turned inwards, or towards the centre of the vein ; in other 

 words, they point in that direction where there was most space for the 

 development of the crystals. Thus each new layer receives the im- 

 pression of the crystals of the preceding layer, and imprints its crystals 

 on the one which follows, until at length the whole of the vein is filled : 

 the two layers which meet dovetail the points of their crystals the one 

 into the other. But in Cornwall, some lodes occur where the vertical 

 plates, or combs, as they are there called, exhibit crystals so dovetailed 

 as to prove that the same fissure has been often enlarged. Sir H. De 

 la Beche gives the following curious and instructive example (fig. 713) 



c a 



e f 



Copper lode, near Redruth, enlarged at six successive periods. 



from a copper-mine in granite, near Redruth.* Each of the plates or 

 combs (a, b, c, d, e, f) are double, having the points of their crystals 

 turned inwards along the axis of the comb. The sides or walls (2, 3, 

 4, 5, and 6) are parted by a thin covering of ochreous clay, so that each 

 comb is readily separable from another by a moderate blow of the ham- 

 mer. The breadth of each represents the whole width of the fissure at 

 six successive periods, and the outer walls of the vein, where the first 

 narrow rent was formed, consisted of the granitic surfaces 1 and 7. 



A somewhat analogous interpretation is applicable to numbers of other 

 cases, where clay, sand, or angular detritus, alternate with ores and 

 veinstones. Thus, we may imagine the sides of a fissure to be encrusted 

 * Geol. Rep. on Cornwall, p. 340. 



