OF UNSTRATIFIED ROCKS AND VEINS. 141 



planes may be seen and examined throughout, in 

 many parts of the Western Islands of Scotland. Nei- 

 ther can it he true of either, if we consider their 

 connexions. If indeed the depth of vertical trap 

 veins cannot in some instances he traced, it only 

 proves that the masses in which they originate, and 

 which form their limits, are innacessihle to us. 



The ramification of veins is a circumstance which 

 requires notice in treating of their dimensions. It 

 is extremely common in granite, but rare in trap; from 

 causes that will be so apparent in the progress of this 

 discussion as to require no specific explanation. 



The position of veins with respect to the horizon 

 is infinitely varied, as it also is with regard to the in- 

 tersected strata. Those of granite are generally ex- 

 ceedingly inconstant, while they also pursue tortuous 

 and intricate directions which defy all attempts to 

 follow them. It is only where they are of large di- 

 mensions, and hold long courses, that they appear 

 straight; and, in these cases, they seem to maintain 

 high angles towards the horizon. But their direction 

 and extent are, in fact, regulated by the magnitude 

 and disposition of the fissures which they occupy, and 

 by the position of the central mass. If, in Cornwall, 

 the prevailing and larger granite veins hold a common 

 course approaching to the parallel, and if the high 

 angles predominate, it only shows that circumstances 

 in the nature and position of the invaded strata, and 

 in the bulk and form of the invading mass of granite, 

 determined this particular disposition in that place. 

 No general rules can be deduced from any fact of 

 this nature- 



With respect to trap veins, it is a remarkable fact 

 that they are rarely congregated into one spot, in mi- 

 nute ramifications; although I have elsewhere shown 



