146 J. W. JUDD ON THE STRUCTURE AND 



eruptions giving rise to the earliest of the lava currents, we have 

 no decisive evidence. It seems probable, however, that it was not 

 the point now occupied by the apex of the hill, biit some adjacent 

 one on the same great fissure — that, in fact, shifting of the volcanic 

 vent along a line of fracture (a phenomenon so frequently displayed 

 in recent volcanoes) took place in a precisely similar manner in Car- 

 boniferous times. 



Considerable intervals must have occurred between some of the 

 earlier eruptions of this volcano, as is shown by the deposits of 

 sandstone and shale with which its products are interbedded. 



Eventually, by gradual elevation, this submarine volcano became 

 a subaerial one, as is shown by the characters of the agglome- 

 rates composing the higher and later portions of its mass*. The 

 changes which were seen to take place in Sabrina and Graham's 

 Isle, and of which we have such unmistakable evidence in the case 

 of Epomeo and Etna, are exemplified in an equally clear manner in 

 the case of the old Carboniferous volcano of Arthur's Seat. 



Nor can we fail to recognize a double cause for this elevation of 

 the volcano above the sea-level -.—first, in the accumulation of the 

 ejected materials around the vent ; and secondly, in the injection of 

 great sheets of liquefied materials below it. 



The extinction of this volcano was probably, as we have already 

 remarked, followed by a certain amount of central subsidence, con- 

 sequent perhaps on the deflection of the liquefied materials beneath 

 it to other points on the same or neighbouring fissures. Such local 

 movements would doubtless greatly contribute towards the preser- 

 vation of the hardest and best-protected masses of the volcanic cone , 

 when, slowly sinking beneath the waters of the Carboniferous sea, it 

 was gradually buried under later sediments. 



Through the long eras of the Secondary and Tertiary, the vast 

 masses of Newer Palaeozoic strata in Central Scotland, with their 

 imbedded ruins of volcanoes, have been gradually assuming their 

 present folded and fractured condition, doubtless in consequence of 

 the operation of those same subterranean forces which in earlier 

 periods were able to force outlets for their violence at the surface. 

 During the same long ages, that equally powerful agency, the circu- 

 lation of surface-waters, acting ceaselessly and contemporaneously 

 with the igneous forces, was sweeping away and redistributing such 

 portions of the mass as were brought by the action of the latter 

 under its sway. Thus, by the constant interaction of antagonistic 

 forces, the final changes in the features and positions of the rocks 

 of Arthur's Seat were brought about. Of these periods, none of the 

 monuments raised at the surface have survived in the central valley 

 of Scotland ; but I am seeking to decipher their strange history, in 

 fragmentary and wondrously preserved records in the adjacent High- 

 lands. 



* I have found remains of plants in the masses of agglomerate forming the 

 apex, of Arthur's Seat. These, however, are too imperfect for identification. 

 As it is possible that they maybe ejected fragments of an earlier-formed deposit, 

 like that of St. Anthony's Chapel, they do not afford any aid in the present 

 inquiry. 



