624 SWELLING OUT OF VEINS. [Ch. XXXVIII. 



with siliceous matter, as Von Buch observed, in Lancerote, the walls of 

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

 which hot vapours had deposited hydrate of silica, the incrustation 

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

 clay or sand, and afterwards reopened, the new rent dividing the argil- 

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

 metals and spars may then be precipitated from aqueous solutions among 

 the interstices of this heterogeneous mass. 



That such changes have repeatedly occurred, is demonstrated by oc- 

 casional 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 superin- 

 tendence, 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, accom- 

 panied by sulphurets of iron and arsenical pyrites, was introduced. 

 Another convulsion then burst open the rocks along the old line of frac- 

 ture, and the first set of deposits were cracked and often shattered, so 

 that the new rent was filled, not only with angular fragments 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 horn- 

 stone 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 catas- 

 trophes 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 fissures 

 only partially filled must naturally offer much feebler resistance along 

 the old lines of fracture than any where else. It is quite otherwise in 

 the case of dikes, where each opening has been the receptacle of one 

 continuous and homogeneous mass of melted matter, the consolidation . 

 of which has taken place under considerable pressure. Trappean dikes 

 can rarely fail to strengthen the rocks at the points where before they 

 were weakest ; and if the upheaving force is again exerted in the same 

 direction, the crust of the earth will give way anywhere rather than at 

 the precise points where the first rents were produced. 



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



