286 



ME. J. S. GARDNER ON THE LEAF-BEDS 



being covered and overlapped by pure flint sand. Such an arrange- 

 ment of material might have resulted from a mountain-stream of 

 velocity sufficient to move rocks of relatively large size along its 

 bed, giving place through changed gradient to a current not exceeding 

 5 or 6 feet per second, and no longer cutting away the basalts. 

 Further depression seems to have converted it into a smoothly 

 flowing river, with a current only capable of transporting grit and 

 sand. The greatest width occupied by its bed does not appear to 

 have exceeded a mile in this part of its course. The peculiarity of 

 these gravels and sands is that they seem composed exclusively of 

 angular fragments of flint derived from Chalk, without any other 

 foreign substance. We may infer from this that the Tertiary Traps 

 had no great extension southward, and I believe, in spite of then- 

 great thickness, that their present limits on that side coincide 

 somewhat with their original confines. A considerable tract of hilly 

 country must have been covered with Chalk. 



At Ardtun the coarsest gravels are much finer than any gravel at 

 Carsaig, and unlike the latter their small pebbles are imbedded in 

 sand. The velocity of the currents which deposited them, even in 

 the swiftest channels, must have been under 3 feet per second, 

 while there were currentless backwaters depositing ooze or fine silt 

 into which leaves of forest trees were eddied aud sank. An increase 

 in the volume of water ensued ; for not only are the higher gravels 

 more massive and of coarser material, but all backwaters and slow 

 currents disappear, and the whole river-bed seems to have been 

 occupied by swiftly moving waters. The transverse section through 

 the gravels is not less than a mile, probably a mile and a half long ; 

 and though the section may be oblique, increasing the apparent 

 width, the position of the lignites, &c, towards Bunessan and the 

 Carsaig section, shows that the actual width was not far short of 

 this. 



The river may not have occupied the entire bed at any one time, 

 but it was evidently one of magnitude when a renewed volcanic 

 outburst filled in its bed with a massive flow of trap. It is interesting 

 to remember that Prof. Geikie recognized the course of another of 

 these Tertiary rivers in the Scur of Eigg, coming from a northern 

 direction (Q. J. G. S. vol. xxvii. p. 309), while the interbasaltic 

 sedimentary beds of Antrim furnish examples of still more con- 

 siderable rivers flowing in the same direction. 



Besides the difference in their coarseness, the gravels of Ardtun 

 are distinguished from those of Carsaig by a large mixture of rolled 

 pebbles of volcanic rocks, which may have come from the direc- 

 tion of Benmore, showing the influx of a tributary stream from a 

 region of acid eruptions which had not been overwhelmed by the 

 Traps. The central region of Mull appears to me in fact to have 

 remained elevated during the Eocene, while the Traps were piled 

 round its sides to a depth of 2000 feet *. A restoration of the 



* This opinion is partly based on a careful survey of Beinn a Ghraig, a ridge 

 of syenite between Benmore and Loch Ba, reaching a height of 1939 feet. The 

 original mass is traversed obliquely by three gigantic dykes of felstone of slightly 



