374 Reviews — Geikie's Ancient Volcanoes of Great Britain. 



Again, " In the Creag nam Favileann (Seamews' Crag) and the 

 gully that cuts its eastern end, likewise in the two singularly 

 picturesque stacks of Dun Mor and Dun Beag, or Big and Little 

 Gull Eocks [PI. XVII, Figs. 1, 2], which here rise from the fore- 

 shore, two distinct platforms of detrital material may be noticed 

 among the basalts. Both of these can be well seen in Dun Mor, 

 about 100 feet high [see Fig. 2]. 



" The lower band, four or five feet thick, is here a rather coarse 

 conglomerate, which lies upon a sheet of scoriaceous basalt that 

 extends up to the base of the Creag nam Favileann. It is directly 

 overlain by another basalt, about 30 feet thick, which dips seawards 

 and forms a broad shelving platform whereon the tides rise and 

 fall. On this stack a second coarse conglomerate, about 10 feet 

 thick, forms a conspicuous band about a third of the height from the 

 bottom ; it is composed mainly of well-rounded blocks of various 

 lavas up to 18 inches or more in diameter, but it contains also pieces 

 of Torridon Sandstone. It is covered by about 60 feet of basalt, 

 which towards the base is somewhat regularly columnar, but passes 

 upward into the wavy, starch-like, prismatic structure." 



The story of Dun Beag is still more interesting and remarkable, 

 but in order to explain its structure properly one ought to have bad 

 at least three figures, figs. 272, 273, and 274; but those who are 

 really interested will get the book and read it for themselves. 



It is these and such like megalithic monuments which enable us 

 to grasp the fact to what an enormous amount of denudation the 

 old volcanic outflows must have been subjected, Eeferring to 

 the effects of denudation Sir A. Geikie writes: — "The original 

 height of the plateau of Mull is shown by the outlier of Ben More 

 to have been at least 3,200 feet. If to this figure we add the 

 portion of the basalt-group submerged under the sea, the height 



will probably be increased by several hundred feet If, 



in the meantime, we suppose the present mean level of the plateau 

 to be 1,000 feet above the sea, the difference between this amount 

 and the assumed original height will be 2,000 feet. If, further, 

 we take the present average rate of degradation of the Mull plateau 

 to be e-oVo of a foot in a year, which has been shown to be 

 probably a fair estimate, then the time required for the lowering 

 of the Mull plateau from its original to its present average level 

 amounts to twelve millions of years. Yet this period, vast though 

 it be, does not carry us back even as far as the beginning of Tertiary 

 time" (pp. 461-2). 



" Among the broad features which soonest arrest attention in 

 a survey of volcanic action is the tendency of volcanoes to range 

 themselves along continental borders or in oceanic islands. This 

 energy in Britain has shown itself along the western or Atlantic 

 margin of the European continent. 



"Moreover the volcanic rocks in Britain are ranged along the 

 greatest length of the group of islands, in a general north-and-south 

 line, from the south of Devonshire to the far Shetlands, and it is 

 always on the western side of the country that they occur. 



" A second and still more remarkable feature in the geological 



