Mae Culloch on Pitchstone. 43 



stratified rocks, these are conformable veins, like those of 

 Arran. 



In every situation except one, where this rock occurs in 

 Scotland, it forms veins, generally so decided in their characters, 

 and intersecting in so marked a manner the accompanying strata 

 at angles, as to permit no doubt respecting their real nature. 

 In a few instances these veins are parallel to the beds in which 

 they lie ; but, in this respect, they differ in no way from the in- 

 truding masses of trap that so often occupy similar positions. 

 These veins are rarely continued for a long space, more com- 

 monly terminating abruptly among the surrounding rocks. 

 The ramification of these veins are so rare that I have hitherto 

 observed it only in one instance ; one of the pitchstone veins of 

 Egg being divided round a mass of chert. 



The only mass of pitchstone in Scotland of which the true 

 position appears at first sight doubtful, is that which forms the 

 Scuir of Egg, which is raiher, however, a substance intermediate 

 between that rock and basalt, and which is at the same time 

 porphyritic. It stands insulated in the form of a narrow irre- 

 gular wall on the surface of a mass of trap. So far it resembles 

 abed as little as it does a vein. But that it is not a bed depo- 

 sited on the trap, nor a stratified rock, is evident from this ; that 

 the same mass of trap lies above the latest of the secondary 

 strata containing coal, into which it also intrudes in the usual 

 manner. If this pitchstone mass were, therefore, a stratified 

 rock, it must be considered not only as later than the latest 

 stratified rocks of which we have any knowledge in Scotland, 

 but posterior also to the trap which succeeds these at a distant 

 interval of time. Under any view, therefore, it is no part of the 

 secondary strata. It is more probably the remains of a vein 

 which has oncfe existed in that trap on which it seems now to 

 stand. The degradation of that rock may easily be imagined 

 to have left the more durable pitchstone in the position it now 

 occupies : and of this degradation, the surrounding rock, which 

 is of a loose amygdaloidal character, gives ample evidence. 

 The columnar structure is no objection to this view, as that 

 structure exists frequently in trap veins. Neither is it an ob- 



