12() OVERLYING AN 7 D TRAP ROCKS. 



Besides the superincumbent masses thus discussed, 

 the trap rocks occur in beds, alternating with, or sur- 

 rounded by the stratified ones ; while I need not dis- 

 tinguish between the primary and secondary strata; 

 as the same appearances are found in both, though 

 much most frequently in the latter. These beds vary 

 in thickness and in frequency of repetition ; forming, 

 at times, mere laminae, at others, considerable masses, 

 but being commonly, both limited and irregular. When 

 carefully examined, and through a considerable extent, 

 they present many peculiarities by which they are dis- 

 tinguished from the associated rocks. Though often 

 appearing to be parallel, that parallelism is either im- 

 perfect or not persistent. They are often, it is true, so 

 situated, that sufficient access becomes impossible ; 

 but, I have described cases in Sky, where even after 

 maintaining their regularity for a mile or more, they 

 have given indications of their peculiar nature ; by in- 

 tersecting the same strata to which they were elsewhere 

 parallel, and then holding a similar course between 

 another pair. In other cases, an apparent parallelism is 

 proved, on a rigid examination, to be fallacious; while 

 in others again, the same vein which intersects a large 

 mass of strata, loses that direction at another point, 

 and runs parallel to them ; thus also producingthe decep- 

 tive appearance of an alternating bed. These alternating 

 masses must therefore be considered as veins holding a 

 course parallel to the strata in which they lie, as in 

 the case of granite, and not as beds deposited in alter- 

 nation with the strata. And this is further confirmed 

 by the communication of an overlying mass with such 

 apparent beds; which are thus shown to be veins in a 

 peculiar position, diverging from it, as the more com- 

 mon transverse veins do frgm similar masses. Such 

 apparent beds also themselves ramify : giving out mi- 



