64-6 Nature of Mineral Veins, 



alumine. But the quantity of lime in felspar, being so ex- 

 tremely small, will not account for large, or comparatively 

 large, formations of calcareous spar; particularly as there are 

 no signs of decomposition in the adjacent felspar. It is pos- 

 sible that minute portions of carbonate of lime may exist 

 between the component minerals of the granite, from which 

 the crystallisation may take place; particularly as I am not sure 

 that the crystals have ever been found in cavities formed ex- 

 clusively in the quartz veins. It is, likewise, possible that cal- 

 careous deposits may, at some period, have existed on the 

 granite, from which the crystallisation may have been formed 

 by infiltration through infinitely minute invisible fissures ; the 

 deposit being afterwards destroyed by arrosion, or other causes. 

 This calcareous spar is sometimes accompanied by sulphur. 

 I have a piece of it from the granites of Chamouni, with 

 bright yellow native sulphur, in the form of a thick coating, 

 here and there imperfectly crystallised. The reverse of this 

 formation of calcareous matter in siliceous rocks occasionally 

 seems to take place, but is usually more easily to be ac- 

 counted for. I remember finding, near Bassano, in Lom- 

 bardy, a piece of compact, hard, fine-grained limestone, with 

 a vein of chalcedonic flint, about a quarter of an inch broad, 

 running through it, apparently so, at least ; for, though im- 

 probable, it is not impossible, that the silex may accidentally 

 have assumed a tabular form (especially as the fragment was 

 not large) before becoming involved in the calcareous matter. 

 Many instances might be given of silex occurring singularly 

 involved in such substances ; but they are generally only 

 accidental juxtapositions. At p. 581. is a notice of " pellucid 

 spar " occurring in a nodule of jasper. The term " spar " 

 is usually applied to calcareous crystals ; but I suppose 

 it here implies quartz : if not, the fact is very remarkable. 



1 remember, in corroboration of the remark that crystals 

 are usually connected with rocks in a state of decay, a bed of 

 clay, formed by decomposing clay slate, near Bangor, in 

 North Wales, containing aggregates of beautiful clear rose- 

 coloured quartz crystals. Of course, when the mineral, as in 

 slate, is a constituent part of the rock, decomposition of the 

 one will occasion crystallisation of the other ; but where, as 

 in granite, it is only a component part, the crystallisation 

 appears to be occasioned by a superabundance of the mineral; 

 for it is neither produced nor attended, neither preceded nor 

 followed, by decomposition. After all, the formation of 

 earthy crystals in a rock of a different nature is hardly more 

 anomalous than that of metallic nests. At p. 582. it is said 

 that veins, having been once fissures, " have since been filled 



