140 ON THE CHARACTERS AND DISPOSITION 



been done in at least the far greater number of in- 

 stances, where the attempts have been properly made. 

 That this may not always be possible, is easily con- 

 ceived : the granite mass may be covered by the strata, 

 and the mass of trap may have disappeared. As trap 

 is also, like granite, a subterranean production, it is 

 easy to suppose that veins of it may, like those of 

 granite, be connected with invisible deep-seated 

 masses. This very fact is proved by phenomena oc- 

 curring in Sky, which will be more amply described 

 hereafter. It is further necessary to observe, that 

 this apparent independence of these veins has oc- 

 curred only where they were of great size; that being 

 the sole condition under which they could exist at 

 great distances from the parent mass. I ought per- 

 haps here to add, that I have traced the sources of 

 all those veins in Scotland which were formerly sup- 

 posed to be independent. It will not be an illegitimate 

 conclusion, that all such veins are processes from 

 masses of those rocks to which they belong in cha- 

 racter and constitution. 



The sizes of these veins are infinitely various, whe- 

 ther we regard their breadth or longitudinal extent. 

 They have been observed to reach to many yards, and 

 to decend from that to the finest filaments. In their 

 length they also vary; but it must be remarked that 

 the courses of trap veins are far more extensive than 

 those of granite. The larger ones of this nature are 

 known sometimes to extend through spaces of many 

 miles. It has sometimes been imagined that they 

 were unlimited downwards; in which case every 

 vein must be a lamina. That however is not the fact 

 in granite, in which every dimension, even of the 

 largest, can sometimes be easily traced. It is equal- 

 ly untrue of trap; as the lateral boundaries of their 



