68 Mb HOPKINS, ON RESEARCHES IN PHYSICAL GEOLOGY. 



to have been filled from below, or the lower part filled from above? 

 Either the one hypothesis or the other appears totally inadmissible, 

 unless we suppose the communication between the upper and lower 

 parts of the vein to have been formerly very much more perfect than 

 at present. This hypothesis would, perhaps, present no very serious dif- 

 ficulty, because it is very possible to conceive the toadstone to have 

 been so imperfectly solidified at the time of the formation of these 

 fissures, as afterwards to diminish their width, by yielding in some 

 measure under the pressure of the superincumbent mass. But if 

 we suppose the portions of the fissure both above and below the 

 toadstone to have been filled either from above or below, ^vhile 

 there existed a wider fissure connecting them through the toadstone. 

 this fissure in the toadstone must also have been filled before its 

 ultimate degree of contraction, in which case it appears almost im- 

 possible that there should not be a much more determinate trace of 

 a vein through the toadstone, than is at present observed to exist. 

 We seem almost necessarily driven in these cases to the hypothesis of 

 some process of segregation or infiltration into fissures previously formed 

 for the reception of the segregated or infiltrated matter. 



^. On the Formation of Granite Veins. 



73. These veins have been described (Introd. vii.) as distinguished 

 in general by the absence of that tendency to rectilinearity and parallelism 

 in their directions which so distinctly characterize the principal mineral 

 veins in each mining district. The fact of these veins being found 

 only at the junction of masses of granite with other masses of dif- 

 ferent mineralogical constitution, has naturally suggested the idea of 

 these veins being veins of injection; the granite being assumed to be 

 of igneous origin. This opinion seems strictly in accordance with the 

 views which we have been developing. The rectilinearity of mineral 

 veins is due, according to this theory, to the predominance of tensions 

 acting in a particular direction, whereas fissures formed in great measure 

 by the hydrostatic pressure of injected fluid matter, in a mass sub- 

 jected to no tension very determinate in its direction, might assume 



