92 PLAJfT-BEDS OP THE BASALTIC FOEMATION OF FLSTER. 



The Ardtun Leaf-bed, Isle of Mull. 



This is most accessible on the sides of a small ravine, and it 

 consists there of a much squeezed, indurated, almost black shale, 

 from two to three feet thick, capped with a film of hard grey mud. 

 Both above and below is hard rock, described by the Duke of Argyll 

 as tuff. It seems to extend some 50 yards east, and a less distance 

 west. Several other varieties of sedimentary rock come in and ac- 

 company it in its eastward extension, among them a strong mass of 

 current-bedded yellowish-white river-sand. A little above it in the 

 ravine is a mass of angular flints and mud, evidently the result of 

 some sudden volcanic flood. Other beds in the ravine may contain 

 vegetable matter, but I only recognized one that could be termed a 

 "leaf-bed." The top film, for about half an inch, is pale drab 

 indurated mud, and is interesting as marking a change in the condi- 

 tions, and possiblj' a volcanic eruption. Less than a foot down is a 

 useful parting of softer matter, which enables considerable blocks of 

 the matrix to be lifted without difficulty. The upper stratum may 

 be described as highly laminated shale, the cleavage-planes being 

 formed by the surfaces of a large-lobed or cordate leaf, while in the 

 lower and less laminated part a small oblong leaf is more sparingly 

 distributed. The whole may have been a fine black fetid silt, such 

 as often results from the overflow of a river when its banks are 

 level. I saw no such regularity as that shown by the Duke in his 

 " pictorial section" of Ardtun. 



(For the Discussioir, see the end of the following paper.) 



-A 



