age of Arthur's seat, Edinburgh. 137 



movements as those which have affected the subjacent rocks of the 

 hill. Thus, standing on the western side of the hill, and looking 

 up at the apex, along the line of the valley of the Hunter's Bog, no 

 observer can fail to notice that the great intrusive mass of the apex 

 presents that pseudo-stratification so common in large igneous 

 masses, and which is, in this case, combined with a tendency to a 

 rudely columnar structure. But the interesting point about the 

 apex of Arthur's Seat is this, that its pseudo-strata are inclined at 

 precisely the same angle as the intrusive sheets of Salisbury Craigs 

 and all the lower rocks of the hill. This interesting circumstance 

 was noticed by Maclaren, and it is, indeed, well represented in one 

 of his diagrams illustrating the structure of the hill (see ' Geology 

 of Fife and the Lothians, 1st edit. p. 14, fig. 6) ; he does not 

 appear, however, in 1839, to have recognized its remarkable signi- 

 ficance. 



Great masses of igneous rocks of all kinds — granites, gabbros, 

 dolerites and trachytes — frequently exhibit this peculiar pseudo- 

 stratification, which, when seen in its most complete form, presents 

 the appearance of more or less curved sheets of rock concentrically 

 arranged. But the parallelism of the pseudo-strata of the apex of 

 Arthur's Seat with the inclined beds all around can scarcely be 

 regarded as accidental ; and we cannot but accept it as an indica- 

 tion that this central intrusive mass has partaken of the movements 

 which have affected all the surrounding rocks*. 



II. In opposition to the view that the contrast in character pre- 

 sented by the stratified tuffs at the base of the hill, and the coarse 

 unstratified agglomerates of its upper part, points to difference of 

 age, I may remark : — 



1. That precisely similar variations are frequently displayed by 

 the products of a single volcano, and sometimes, indeed, by different 

 portions of the materials thrown out during the same eruption. This 

 fact is a familiar one to all who have studied the structure of the 

 volcanoes of the Campi Phlegrsei, and is even still more strikingly 

 illustrated in those of the Island of Ischia. The nature of the volca- 

 nic agglomerates is evidently determined, in the first place, by the 

 characters, dimensions, and condition of the fragments falling at 

 any point ; and in the second, by the circumstances of their be- 

 coming agglutinated where they fall, of their being swept along 

 in debacles of mud (lave d'acqua), or to their being triturated and 

 sorted by waves and currents — according as they are deposited on the 

 land or under water. 



2. That the coarse agglomerates and stratified ashes of Arthur's 

 Seat pass into one another by the most insensible gradations, and 

 that in some cases it is impossible to decide to which class a particular 



* 1ST ear the celebrated f ossilif erous locality of Dura Den, in Fif eshire, a quarry 

 opened in an intrusive mass of dolerite, of probably the same age as the apex 

 of Arthur's Seat, exhibits this pseudo-stratification in a remarkably similar 

 manner. As the mass has slowly cooled, a series of parallel slightly curved 

 planes of division have been produced in its mass, and the mass now presents 

 a pseudo-stratification combined with a rude columnar structure which is almost 

 the exact counterpart of that seen at Arthur's Seat. 



