136 J. W. JUDD ON THE STRTJCTUEE AND 



place during that period in Ayrshire and Dumfriesshire. I shall 

 not stay to criticise the supposed proofs of the Permian age of these 

 volcanic rocks in the south-west of Scotland, though they are far 

 from heing complete and conclusive. But I may ask, is it at all 

 in accordance with what we know of the physical and palseontolo- 

 gical relations of the Carboniferous and Permian formations to 

 admit the probability of such a grand series of operations as that 

 to which we have referred having taken place in the interval be- 

 tween them, while so little change has been produced in the infi- 

 nitely longer subsequent periods ? 



Such, then, are the a priori difficulties in the way of our acceptance 

 of the two periods of eruption of the rocks of Arthur's Seat. Let 

 us now inquire if this violent hypothesis be really necessary for the 

 explanation of the structure of the hill. That such is not the case, 

 I propose to demonstrate, 



First, by pointing out that the supposed proofs of the second 

 period of eruption break down upon re- examination ; and 



Secondly, by showing that all the phenomena of the district are 

 capable of much more simple and easy explanation on the hypo- 

 thesis of a single series of volcanic outbursts. 



The grounds upon which a second period of volcanic eruption has 

 been inferred in the case of Arthur's Seat are fourfold :- — 



1. The vertical position of the central column of lava which con- 

 stitutes the apex of the hill, when contrasted with the uptilted beds 

 at its base. 



2. The difference of character between the stratified tuffs in the 

 lower part of the hill and the unstratified agglomerates of its higher 

 portion. 



3. The unconformity supposed to exist between the upper masses* 

 of lava and agglomerate and the lower series of sandstones, tuff- 

 beds, lavas and intrusive sheets. 



4. The position of an alleged " neck " from which the great mass 

 of lava capping the Lion's Haunch is supposed to have flowed. 



I shall now proceed to show that neither of these supposed proofs 

 of a second period of eruption will bear the test of careful exami- 

 nation. 



I. "With regard to the supposed vertical position of the great 

 central column of lava, I may remark that the phenomena so ad- 

 mirably described by Maclaren — namely, the manner in which 

 the basalt of the apex can be traced, passing down through, and 

 giving off veins into the mass of agglomerates — while they demon- 

 strate the lava to be an intrusive column, do not prove its vertical 

 position. There is, in fact, nothing whatever in the appearances 

 described in any way inconsistent with the supposition that this 

 intrusive column, originally vertical or nearly so, may by its subse- 

 quent movement, in common with all the surrounding rocks, have 

 been thrown some 20° from its original position. 



And, on the other hand, there are features in the structure of 

 this lava column which it is very difficult indeed to understand, 

 unless we accept the conclusion that it has partaken of the same 



