772 



SUCCESSIVE FILLING UP OF VEINS. [Ch. XXXVIII, 



posits have accumulated, we may next consider the proofs of their 

 having been filled gradually and often during successive enlargements. 

 I have already spoken of parallel layers of clay, quartz, and ore. 

 Werner himself observed, in a vein near Gersdorff, in Saxony, no less 

 than thirteen beds of different minerals, arranged with the utmost 

 regularity on each side of the central layer. This layer was formed 

 of two plates of calcareous spar, which had evidently lined the oppo- 

 site 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 walls of the rent on each side are the oldest of 

 all. If they consist of crystalline precipitates, they may be explained 

 by supposing the fissure to have remained unaltered in its dimen- 

 sions, while a series of changes occurred in the nature of the solu- 

 tions which rose up from below; but such a mode of deposition, in 

 the case of many successive and parallel layers, appears to be excep- 

 tional. 



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 the direction where there was space for the de- 

 velopment of the crystals. Thus each new layer receives the impres- 

 sion 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. 765) 



c A 



e f 



Copper lode, near Eedruth, 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) is 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 



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



