Ch. XXXYin.] SWELLING OUT OF VEINS. 773 



the hammer. 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 many other 

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

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

 ed with siliceous matter, as Yon Buch observed, in Lancerote, the 

 walls of a volcanic crater formed in 1731 to be traversed by an open 

 rent in which hot vapors had deposited hydrate of silica, the incrusta- 

 tion nearly extending to the middle.* Such a vein may then be filled 

 with clay or sand, and afterwards reopened, the new rent dividing the 

 argillaceous deposit, and allowing a quantity of rubbish to fall down. 

 Various metals and spars may then be precipitated from aqueous solu- 

 tions among the interstices of this heterogeneous mass. 



That such changes have repeatedly occurred, is demonstrated by 

 occasional cross-veins, implying the oblique fracture of previously 

 formed chemical and mechanical deposits. Thus, for example, M. 

 Fournet, in his description of some mines in Auvergne worked under 

 his superintendence, observes that the granite of that country was first 

 penetrated by veins of granite, and then dislocated, so that open rents 

 crossed both the granite and the granitic veins. Into such openings, 

 quartz, accompanied by sulphurets of iron and arsenical pyrites, was 

 introduced. Another convulsion then burst open the rocks along the 

 old line of fracture, and the first set of deposits were cracked and often 

 shattered, so that the new rent was filled, not only with angular frag- 

 ments of the adjoining rocks, but with pieces of the older veinstones. 

 Polished and striated surfaces on the sides or in the contents of the 

 vein also attest the reality of these movements. A new period of 

 repose then ensued, during which various sulphurets were introduced, 

 together with hornstone quartz, by which angular fragments of the 

 older quartz before mentioned were cemented into a breccia. This 

 period was followed by other dilatations of the same veins, and other 

 sets of mineral deposits, until, at last, pebbles of the basaltic lavas of. 

 Auvergne, derived from superficial alluviums, probably of Miocene or 

 older Pliocene date, were swept into the veins. I have not space to 

 enumerate all the changes minutely detailed by M. Fournet, but they 

 are valuable, both to the miner and geologist, as showing how the 

 supposed signs of violent catastrophes may be the monuments, not of 

 one paroxysmal shock, but of reiterated movements. 



Such repeated enlargement and reopening of veins might have been 

 anticipated, if we adopt the theory of fissures, and reflect how few of 

 them have ever been sealed up entirely, and that a country with fis 

 sures only partially filled must naturally offer much feebler resistance 

 along the old lines of fracture than anywhere else. It is quite other 



* Principles, chap, xxvii. 8th ed. p. 422. 



