ARRAN. GEOLOGY. PITCHSTONE. 419 



It is unnecessary to describe the several intersections 

 which these veins undergo from the trap veins amongst 

 them ; as they offer no peculiar interest, and must be 

 sufficiently understood from what has already been related : 

 but before finally quitting this subject, a few remarks 

 on the probable geological nature of this rock will not 

 be superfluous. 



In every situation but one in which pitchstone has yet 

 occurred in Scotland, it is found in the form of veins.* 

 It may perhaps be said that the horizontal mass at Cory- 

 gills is a stratum, but the circumstance of position alone 

 does not justify that conclusion. If that alone were 

 sufficient to distinguish a stratum from a vein, no hori- 

 zontal or conformable veins of trap could exist, whereas 

 they have been proved to be common. It may perhaps 

 be safely asserted that in alnuost every instance where a trap 

 rock in a horizontal position is followed by a stratified 

 rock, it is an intruding mass, and therefore a vein ; and 

 that the only instance in which trap can be viewed as 

 a bed, is where it forms an overlying mass ; since in no 

 instance is there an example of any rock deposited on 

 this substance, which, on the contrary, appears invariably 

 to be itself the latest of all deposits. If there were no 

 other points of resemblance between pitchstone and trap, 

 the following fact alone would be sufficient to prove their 

 analogy, and to establish the true nature of those masses 

 which are conformable to the strata in which they 

 lie. They are found in the secondary strata, in gra- 

 nite, and in the trap rocks ; occupying thus a variety of 

 position of which there is no analogous example except 

 in the rocks of this latter family. No substance decidedly 



* The exception to which I here allude is that of the ScnirofEgg, 

 formerly described ; but if it is an overlying mass it is plainly no obstacle 

 to the supposed posteriority of pitchstone ; it rather tends to confirm that 

 view by strengthening the analogy between this substance and the trap 

 rocks. It is not improbable that it may have been a vein in the trap on 

 which it now stands ; a notion strengthened by it's peculiar shape, and 

 by the evident waste of the surrounding rock. 



