Ch. XV.] OF THE PRIMARY ROCKS. 361 



slate, for instance, is often distinct, and marked by an open 

 seam or joint, yet it is no less true that such a separation 

 does not always exist, the vein and the rock forming indivi- 

 dual concretions, in which both are firmly and intimately 

 united : and this is best observed between the included por- 

 tions of slate and the substance of the vein ; as in such posi- 

 tions decomposition is not so energetic, the percolating water 

 generally following the line of junction between veins and the 

 main mass of rock. 



Now it may be conceded, that fissures would probably be 

 effected in the direction of one or other of the great lines of 

 structure, in preference to a disruption of the solid rock : 

 but it cannot be so readily allowed, that the substance of 

 veins introduced into such fissures could unite with the 

 already consolidated materials of the rocks, so as jointly to 

 compose the same concretions ; that is, that one portion of a 

 concretion has been formed at one period, and another at a 

 different time long subsequent to the former. 



Concerning the composition of veins, if it be admitted that 

 quartz, carbonate of lime, and some other minerals, can be 

 introduced into veins in a state of solution ; yet it will not, we 

 presume, be supposed a possible event, that granite, slate, and 

 other rocks could be deposited in fissures by a similar pro- 

 cess : and yet the true veins of Cornwall generally contain no 

 inconsiderable proportions of the adjacent rocks ; and as these 

 vary in nature, so do the rocky parts included in the veins, 

 as well as the other mineral contents. Oh ! but, say the 

 advocates of the mechanical origin of veins, these portions of 

 rocks are neither exudations nor depositions, but fragments 

 detached from the adjacent mass, which explains their re- 

 semblance in composition. Can any one who has maintained 

 such an opinion possibly have examined the position of these 

 insulated portions of rock, or even reflected on their nature ? 

 for it appears to be a most self-evident proposition, that these 

 pieces, whether detached during the disseverment of the solid 

 rock, or during the violent injection of the matrix into the 



