LUNGA. GEOLOGY. 



graywacke; alternating every where irregularly with all 

 the other rocks before described. 



As these beds occur, even in greater variety, in the 

 remaining islands of this group, and will again necessarily 

 come under review, it is here unnecessary to enter into 

 any further details respecting them. 



Numerous trap veins are seen traversing the strata 

 of this island. They have no certain direction, being 

 in some places erect, in others inclined ; at one time 

 intersecting the beds in angular directions, at another 

 insinuating themselves in a parallel course between their 

 laminae. In some cases, the same vein will be found 

 to occupy both positions, changing its course from a 

 transverse to a parallel one. The deep and perfect 

 sections of the rocks, both here and in Scarba, enable 

 us to trace distinctly those arrangements in the veins, 

 which, in most instances, where the surfaces only admit 

 of examination, are nearly as much matter of induction 

 as of observation. It is from those veins which are 

 entirely or in part conformable to the planes of the 

 strata, that we are enabled to understand the manner 

 in which the character of the including rock has affected 

 the form and disposition of the vein. Where its course 

 lies at angles with the direction of the strata, it is ge- 

 nerally straight and persistent, while its breadth is equal 

 throughout ; as if the violence and suddenness with 

 which the original fissure was formed, had been such 

 as to disregard all impediments arising from inequalities 

 of hardness ; these bearing no proportion to the force 

 exerted in the separation. Where, on the contrary, 

 the fissure has taken place in the direction of the 

 beds, the vein will often be found unequal in thickness, 

 and sometimes curved, or even tortuous ; from the 

 greater facility with which these have yielded in that 

 direction, and from the inequalities originally existing 

 in the disposition of the laminae which by their 



