age of Arthur's seat, Edinburgh. 143 



If any such doubt did remain in the mind of an observer, con- 

 cerning the relations of the several phenomena we have referred to, 

 they would be at once dispelled by a study of the structure of 

 Arthur's Seat ; for here we find all the appearances described 

 associated with one another in the same mass. 



That in Arthur's Seat and the Calton Hill, with the similar volcanic 

 masses of the Castle Rock, Craiglochart, Dalmahoy Craigs, Kirk- 

 newton, &c, we have an indication of one of the lines of subter- 

 ranean fissure running in a N.E. and S.W. direction like those of 

 Forfarshire, no one can doubt. In the doleritic sheets of the St. 

 Leonard's, Salisbury, the Bog, and Girnal Craigs we have manifes- 

 tations of subterranean volcanic action precisely analogous to those 

 so well exhibited in Fife; while in the lava currents, tuffs, and 

 agglomerates of the central hill, the Lion's Haunch, Dunsapy, and 

 Whinny Hill, we recognize the exact counterpart of similar sub- 

 aerial volcanic products scattered all over the area of the Lothians. 



The apparent complexity in the positions of the rocks composing 

 Arthur's Seat, which at first led to the hypothesis of two periods of 

 eruption separated by an interval of enormous duration, will cease 

 to create surprise, if we bear in mind the nature of the actions to 

 which these rocks owe their origin, and the vast and wonderful 

 system of changes which they have since undergone. 



Not a little of the difficulty experienced in seeking to give an 

 explanation of the structure of Arthur's Seat has resulted from 

 attempts to identify different masses of lava at several parts of the 

 hill. If we reflect on the similarity presented in many cases by 

 the different outbursts from the same volcano, the hopelessness of 

 such a task will at once become manifest. On the other hand, 

 when we find the higher portions of a volcanic cone composed of 

 the coarsest agglomerates, while the most finely stratified ashes 

 occur in its outer slopes, the necessity for referring these, in the 

 case of Arthur's Seat, to two widely separated periods will cease to 

 be felt. 



With regard to the present positions of the rock-masses of Arthur's 

 Seat, a careful study of them indicates that they have resulted from 

 four distinct sets of causes: — 



First. The original positions assumed by the outflowing lavas as 

 they issued from the volcanic vent, and of the projected fragmentary 

 materials as they fell around, or were washed down the sides of the 

 cone. 



Second. Changes produced by the injection of masses of igneous 

 rock in sheets (that of Salisbury Craigs is in places 80 feet thick !) 

 or dykes. 



Third. Changes produced by a central subsidence in the volcanic 

 vent, of which we have so many evidences both in ancient and 

 modern volcanoes. Of such movements I believe I was able to 



geologists. I have, however, recently obtained proof, in a fragment of Carboni- 

 ferous strata in Morvern, in Argyllshire, marvellously preserved under masses 

 of Secondary and Tertiary rocks, that parts, even of the Highlands, must have 

 been at one time covered by Carboniferous rocks. 



