384 SKY. GEOLOGY. OVERLYING ROCKS. 



colour and composition; while in others the separate sub- 

 stances lie irregularly intermixed without gradation ; the 

 contact of the two being such as not to indicate the 

 relative priority or posteriority of either. The transition, 

 where it exists, is formed by means of a blue compact 

 felspar, which first replacing the yellow felspar of the 

 syenite, is finally converted into greenstone or into augit 

 rock by the acquisition of hornblende or augit. 



Among the remaining overlying masses of trap that seem 

 worthy of a distinct notice is that which occurs near Swish- 

 nish point, which, though not very extensive, affords an 

 interesting and distinct section of its structure and of its 

 connexions with the subjacent strata, as the accompanying 

 drawing will more fully explain.* Connected with it is 

 one large process or vein an hundred feet or more in 

 thickness, cutting through the strata and lost in the sea, 

 affording an example of the double connexion which trap 

 has with the stratified rocks. At some little distance 

 from this, numerous smaller vertical veins descend in the 

 same manner, cutting through the subjacent strata. This 

 fact illustrates the origin of trap veins, at least in certain 

 cases, even more distinctly than the appearances just 

 mentioned on the eastern coast. It is probable that 

 they are in most instances connected with a principal 

 mass, even where we have not the means of tracing 

 them to their sources ; while it may happen in some, 

 that the masses with which they have once been con- 

 nected, have disappeared in the progress of time, leaving 

 behind those parts only which were protected by the sur- 

 rounding rocks. The ultimate removal of the overlying trap 

 of Swishnish, a circumstance far from improbable, would 

 thus leave the veins in question remaining independent, and 

 thus perhaps induce future geologists to attribute to a 

 protrusion from below that which may equally have 

 entered from above; though it is not necessary, even 



* Plate XIV. fig. 5. 



