140 J. W. JUDI) ON THE STRUCTURE AND 



identification could be relied upon, it would, of course, entirely 

 dispose of the idea that the two masses form parts of deposits of 

 widely different ages ; for the rock below Duddingston churchyard 

 is clearly one of the contemporaneous lavas in the Lower Calciferous 

 Series. I cannot, however, help feeling that in the several expla- 

 nations which have been given of the rocks of Arthur's Seat, the 

 local and isolated character of many of the masses of lava have not 

 been sufficiently borne in mind when attempts have been made to 

 identify them by means of their petrological characteristics in dif- 

 ferent parts of the hill. Thus, I regard the mass of basalt capping 

 the Lion's Haunch, and that of porphyrite which forms the slope 

 leading down from the head of St. Anthony's Valley to Dunsapy 

 Loch, as lava streams that have flowed down the slope of a cone, 

 which, after the manner of modern volcanoes, was doubtless partially 

 destroyed and reformed many times in the course of its history. The 

 present position and prominence of these masses I am disposed to 

 refer simply to their power of resisting denudation, considering 

 them merely eminences capped by portions of hard lava streams 

 analogous to the plateaux of Ischia and the Auvergne, and the 

 Scur of Eigg, though on a much smaller scale. 



Having shown that the hypothesis of a double series of erup- 

 tions at Arthur's Seat is beset by such grave (I may say, insuperable) 

 difficulties, and that the supposed proofs of it break down on re-exa- 

 mination, let us now proceed to inquire if any simpler theory is 

 capable of satisfying all the conditions' of the case. 



It has been already remarked that, in studying the volcanic vent 

 of Arthur's Seat, it is desirable to make comparisons, at every stage 

 of the inquiry, with the numerous similar examples of igneous out- 

 bursts of the same age in Porfar, Fife, and the Lothians. These 

 present us with a series of Carboniferous volcanoes in every con- 

 ceivable stage of dissection by denudation. 



The Highlands and Borderlands of Scotland are constituted by 

 rocks of the same age, but in very different stages of metamorphism. 

 The great central valley situated between these two mountain- 

 groups composed of Lower Palaeozoic strata, is occupied by a series 

 of Newer Palaeozoic formations, the positions of which, in relation to 

 the older rocks lying to the north and south of them respectively, 

 appear to be due, in great part at least, to the action of great N.E. 

 and S.W. faults, which have let down the masses of newer strata 

 between those of the older ones. 



A new light was thrown upon the structure of the central valley 

 of Scotland when Sir Charles Lyell, nearly fifty years ago *, showed 

 that the Newer Palaeozoic strata of the area are bent into a number 

 of anticlinal and synclinal folds, the direction of the axes of which 

 are parallel to the great boundary faults. Later observations, espe- 

 cially those of Charles Maclaren, Mr. Milne-Home, Mr. Powrie, and 

 the officers of the Geological Survey, have shown that the district is 

 also traversed by very numerous faults, maintaining the same N.E. 

 and S.W. direction as the great bounding faults and the axes of 

 * Trans. Geol. Soc, 2nd sei\ vol. ii. pp. 73-96, plates x. and xi. 



