CANNA. DESTRUCTION OF ROCKS. 46' 1 



this process ; so interesting in a chemical view, and so 

 important in its results, from the extensive changes which 

 the surface of the earth undergoes, both from its accumu* 

 lated effects and from their remoter consequences. 



The appearance in question is most conspicuous in the 

 breccia that occupies the lowest situation on some parts 

 of the shore ; but is not limited to that rock, since it also 

 occurs in the prismatic trap and in the amygdaloid.* The 

 waste of these rocks from the action of the atmosphere 

 is considerable, and the indications of it are .visible in 

 many other cases besides those of the .detached masses 

 so remarkable on the shore of Sandy Isle and : on the 

 eastern side of Canna. But where the high wa^er mark 

 commences, the indications of which on rocky shores 

 are almost always sufficiently marked by the Lepas Bala- 

 nus, there is- a sudden stop" to the wasting process, which, 

 although it may vary in the different kinds of trap, is 

 generally visible every where. In consequence of this, 

 a kind of platform is seen at low water, skirting the bases 

 of the cliffs ; often reaching to a great extent, and thus 

 indicating the different degrees of rapidity with which 

 those parts of the rock exposed to the air only, and those 

 subjected to the vicissitudes of sea and air together, have 

 undergone decomposition. In some instances, where 

 detached turriform or pinnacled rocks are seen, the ap- 

 pearance is peculiarly remarkable, since they seem as 

 if they had been erected on solid platforms, the flat sur- 

 faces o;f which denote the level of the sea at high water. 

 Contemplating the rapid waste of these rocks, it is easy 

 to look forward to a time when the pinnacle shall disap- 

 pear, and no trace of it remain but the half sunk rock, 

 breaking at high water and denoting its former place. 



There can be no question respecting the nature and 

 existence of this appearance, since it occurs in other 

 places besides Canna; but as yet I have had no oppor- 



* Plate XIX. fig. 4. * 



