AGE OP AKTHTJIt's SEAT, EDINBURGH. 135 



may be discriminated from that forming the apex of Arthur's Seat ; 

 while the porphyritic dolerite with large grains of olivine and 

 crystals of augite, seen under Duddingston churchyard, is so pre- 

 cisely similar to that capping the Lion's Haunch, that Maclaren 

 supposed them to have been parts of the same lava current*. 



In explaining the structure of the eastern part of Arthur's Seat, 

 Professor Geikie introduced some changes into the original theory of 

 Maclaren, which, however necessary they may appear, must be 

 admitted to increase very greatly the antecedent improbability of 

 the hypothesis of the two distinct periods of volcanic outburst f. 

 According to the amended explanation suggested by Professor Geikie, 

 the outbursts in both the older and newer periods must have com- 

 menced with the emission of basalts, and have been continued by 

 that of porphyrites, the successive products of either period being 

 admitted to be severally undistinguishable in character. 



The series of events which we are thus required to believe took 

 place in this district is therefore as follows : — 



A. At the point where the Arthur's-Seat group of hills now 

 rises, a series of volcanic eruptions occurred during the Lower-Calci- 

 ferous-Sandstone period, commencing with the emission of basaltic 

 lavas and ending with that of porphyrites. 



B. An interval of such enormous duration supervened as to 

 admit of: — 



a. The deposition of at least 3000 feet of Carboniferous strata. 



b. The bending of all the rocks of the district into a series of 



great anticlinal and synclinal folds. 



c. The removal of every vestige of the 3000 feet of strata by 



denudation. 



C. The outburst, after this vast interval, of a second series 

 of volcanic eruptions upon the identical site of the former ones, 

 presenting in its succession of events 'precisely the same sequence^ 

 and resulting in the production of rocks of totally undistinguishable 

 character. 



Are we not entitled to regard the demand for the admission of 

 such a series of extraordinary accidents as evidence of the antecedent 

 improbability of the theory ? And when we find that all attempts to 

 suggest a period for the supposed second series of outbursts have 

 successively failed, do not the difficulties of the hypothesis appear to 

 be overwhelming ? As we have already seen, the opinions of the Ter- 

 tiary and Secondary epochs as the date of the second series of erup- 

 tions have both been abandoned ; and now they are doubtfully 

 referred to the Permian, on the ground that volcanic eruptions took 



* I do not propose to digress from the main object of this paper in order 

 to discuss the true nature of the rocks of Arthur's Seat, as deduced from their 

 chemical constitution and petrological characters. In a valuable paper recently 

 published in this Journal (vol. xxx. p. 553), Mr. Allport has described several 

 of them. Here I shall content myself with referring to the rocks of the 

 basaltic class as " dolerites " or " basalts," and those of the trachytic class 

 as " porphyrites." The latter were called porphyries by the older authors. 



t The Geology of the Neighbourhood of Edinburgh (1861), p. 124, and note ; 

 Horizontal Sections of the Geological Survey, Sheet 1 (1862) ; &c. &c. 



