134 J W. JUDD ON THE STRUCTURE AND 



the structure of the western and basal portions of this group of hills 

 all geologists are now perfectly agreed. 



The foundations of Arthur's Seat and Salisbury Craig consist of 

 rocks belonging to the lower part of the Calciferous Sandstone series, 

 sandstones and shales of estuarine origin, alternating with deposits 

 of stratified volcanic tuffs, and streams of basaltic and felspathic 

 lava. Between these lower beds of sandstone, shale, &c. at least 

 three great sheets of doleritic lava have been injected, which, in 

 consequence of their greater relative capability of resisting denuding 

 influences, now constitute the three bold escarpments known as the 

 St. Leonard's, Salisbury, and Bog Craigs (the latter of which is also 

 called the Dasses). Above this foundation of sandstones and shales, 

 with interbedded and intrusive volcanic materials, a somewhat com- 

 plicated mass of rocks rises, consisting of lavas, in currents * and 

 dyke-like masses, with layers and piles of volcanic agglomerates. 



All these rocks at the base of the Arthur's-Seat hills have a dip 

 to the E.N.E. averaging about 20° ; and nothing can be clearer than 

 the fact that they form a part of one of the great anticlinal folds 

 into which the whole of the strata of this district have been thrown. 

 Another fact which has been recognized by all geologists who have 

 studied the district is, that the lower rocks of Arthur's Seat must 

 have been buried under at least 3000 feet of the higher Carboniferous 

 strata, similar to those which are still seen occupying the bottoms 

 of the synclinal troughs on either side of them, and which have 

 evidently been removed from the summits of the anticlinals by 

 denudation. 



Such, then, are the points on which all are agreed. Let us now 

 turn to those conclusions which we regard as open to question. 

 According to the generally received hypothesis of the structure of 

 the hill, the higher masses of rocks constitute a great unconformable 

 " cake," of far later date than the basal portions of the hills, and 

 laid down upon these subsequently to their upheaval and denudation. 

 The grounds of this conclusion we shall now proceed to examine. 



At the outset of this portion of the inquiry, however, we may 

 notice the remarkable fact that no geologist has succeeded in point- 

 ing out any distinction whatever, either in their penological cha- 

 racters or their state of preservation, between the lavas of the 

 basal and upper portions of Arthur's Seat respectively. The basalts 

 of Whinny Hill, for example, present no features whereby they 



* One of the lava streams in this portion of the series affords an interesting 

 example of a most characteristic volcanic structure, which, strange to say, appears 

 to have been hitherto quite overlooked. I refer to the mass called the Long 

 Bow, which overhangs St. Anthony's Valley or the Dry Dam, in which we find 

 a basaltic lava current, the lower portion of" which is divided into rude vertical 

 columns of large diameter (the base presenting the usual highly vesicular or 

 scoriaceous characters), while the upper part of the same current is made up of 

 smaller curved and interlacing columns passing in places into an amorphous 

 mass. Though less regular and pronounced, the structure of this lava stream 

 is identical with that which gives rise to the beauties of Staffa, and is so fre- 

 quently exhibited in the Auvergne and other volcanic districts. The nature 

 and origin of this structure has been discussed by Mr. Scrope. See ' Volcanos,' 

 p. 94. 



