ARDTUN LEAP-BEDS. 



105 



upward into a mass of indurated gravel seven feet thick. In the ravine the gravel bed 

 is externally as black as the lava, but fresh surfaces of the rock, shattered by blasting, 

 showed a clear steely, French-grey matrix, with pure white flints so firmly cemented that 

 the fractures had invariably passed through rather than round them. Here and there 

 were stones of a cherry-red, and closer inspection revealed that many pebbles of almost 

 the colour of the matrix had likewise been broken through. This pudding-stone, or 

 indurated gravel, passes upward gradually into purer sand, eight feet thick, and 

 comparatively soft and fissile at the top. The final parting between the sedimentary 

 beds and the lava above is a rubble similar to that at the bottom. 



This ravine proved, however, by no means the best spot, either for observation or 

 collecting, though the most accessible. It appears in fact to be due to the weathering out 

 of a dyke, now decomposed. The pressure of the upward passage of the incandescent mass 

 has squeezed and distorted the plant beds and discoloured the gravel. The latter appears 

 so altered as to have been mistaken for a volcanic agglomerate, but closer examination 

 shows that it has the bedding of true river-gravels, and that all its materials are water- 

 worn. Eighty yards farther east it has thickened to twenty feet, and the sand preserves its 

 original colour. The current-bedding is beautifully shown, together with the drifts of 

 shingly gravel and some small round boulders, so that no one could fail to recognise at the 

 first glance its fluviatile origin. In addition to the leaf-beds at the ravine already described, 

 which are continued unchanged, we find underneath, in descending order, a second gravel 

 bed, of finer grain, three feet thick ; two feet of grey clay with larger leaves in the rust- 

 coloured partings ; six inches hard laminated gritty sandstone with leaves ; three inches 

 hard lavender and buff cement-stone with very well-preserved leaves ; one foot grey clay, 

 with a thin layer at the base, literally choked with leaves in fine preservation, but very 

 difficult to remove ; six to nine inches clunch, inclined to be concretionary, with rootlets ; 

 and lastly, a thin layer of carbonaceous matter and grey clay filling in the inequalities of 

 the old lava surface. There is a considerable variety of leaf in the lowest leaf-bed, but the 

 most striking and abundant are those of Ginkgo. In addition to the dyke now weathered 

 out, the Head has been traversed by an extensive sheet of Basalt, which has forced a devious 

 way upward at a low angle through the series ; and this also, owing to denudation, comes 

 to the surface at the ravine. It is twenty to thirty feet thick, readily distinguished by its 

 superior hardness and density, and finer grain ; and it obliterates the sedimentary beds for 

 some distance where it crosses them. The gravel, however, soon reappears ; and where last 

 seen, at a distance of 700 yards from the ravine, is still twenty feet thick. The fluviatile 

 series is therefore of considerable extent, and its importance has been much underesti- 

 mated. It is now known to occur at Carsaig on the other side of the Ross of Mull, at 

 Bourg, and in Loch Na Kael, and it will be interesting to trace its limits in other 

 parts. Though the gravels are unfossiliferous at Bourg, a large tree,^ with a trunk five 



1 The wood is carbonised, and charred. Mr. Healy has kindly examined it microscopically and pro- 

 nounces it Coniferous ; and Mr. Carruthers thinks it might possibly be a trunk of Podocarpus, 



