222 Professor Gustav Bischof on the 



deposit in them the substances which it held in solution. If 

 but the narrowest fissure were formed in a rock, its filling, 

 or the formation of vein -masses, could begin. If the narrow 

 fissure were filled, and no splitting or widening of the ori- 

 ginal cleft occurred, then was the formation of a vein com- 

 pleted. In this manner, for example, were formed the small 

 veins of quartz which so frequently traverse clay-slate. If 

 the deposition of vein-masses was the result of an exchange 

 with the constituent parts of the adjoining rock, then a widen- 

 ing of the fissure would naturally be produced, or it would at 

 least result that the vein-masses should penetrate into the 

 adjoining rock. If the cause of the Assuring of the crust of 

 the earth continued in operation during the formation of the 

 vein-masses, and should it have been, as, without doubt, gene- 

 rally was the case with the crystalline rocks, the continued 

 cooling, and the consequent contraction which produced this 

 splitting, then, on the supposition that the influx of the watery 

 fluid did not cease, the deposition of the vein-masses would at 

 the same time go on. 



Either the continued widening of the cleft kept pace with 

 the filling or preceded it, in which cases an open chan- 

 nel always remained in the middle of the cleft, and the ad- 

 hesion of the oldest portions of the vein with the adjoining 

 rock was not interrupted ; or the widening of the cleft took 

 place by fits and starts, so that, after the filling was com- 

 pleted, a new cleft was formed, which encountered those 

 portions of the vein-mass or of the adjoining rock which 

 were kept together by the smallest degree of cohesion or ad- 

 hesion. In this latter case the vein-mass, formed slowly and 

 by gradual precipitation, and which was therefore very com- 

 pact, and of difficult frangibility, would present greater re- 

 sistance than that ofi^ered by the adjoining rock, which had 

 probably become soft owing to decomposition, or by the 

 agency of the water which had penetrated into it. Hence 

 it happened that fragments of the adjoining rock which ad- 

 hered closely to the vein-mass were separated from it, and 

 were surrounded by a newly commenced precipitate from the 

 circulating liquids. If the precipitation was produced by 

 mutual exchange, then it would be favoured by the new 



