276 



JIB. J. S. GABDNEB OIsT THE LEAP-BEDS 



and then a bed of black crumbly shale (/), 2 feet 4 inches thick, crowded 

 with leaves*. The lowest part of this is greatly squeezed, and desti- 

 tute of recognizable fossil leaves t, but a few succeeding layers con- 

 tain innumerable specimens of a simple ovate leaf, the Rhamnites of 

 Edward Forbes %, then some squeezed layers with decomposed leaves 

 and Equisetum, and, lastly, a layer made up almost entirely of the 

 large leaves of Platanites, Forbes. Above this, but separated by a per- 

 fectly sharp plane, is a bed(e), one foot thick, of very dark, intensely 

 indurated whinstone or rag, originally fetid mud, in which immense 

 leaves of Platanites are rolled and folded together with broken and 

 mostly very macerated fronds of Onoclea (Filiates) hebridica, Forbes, 

 broken stems of Equisetum, and occasional twigs of Taxus (Taxites) 

 Carnpbellii, Forbes. Magnificent specimens from this layer were 

 obtained and are now in the British Museum, one, not far short of 

 a square yard in surface, exhibiting specimens of all these except 

 the last. Another plane separates it from a similar bed, but with 

 few fossils, which passes gradually upward into the overlying gravel 

 (d), at this point only 7 feet thick ; this in turn passes into some 

 fissile sand (c), becoming softer at the top and, in all, 8 feet thick. 

 The Duke's first leaf-bed is at this horizon, though I failed to find it 

 fossiliferous. The parting between this and the trap above is carbo- 

 naceous rubble, similar to that at the base. 



The gravel-bed is of the greatest interest, and its composition has 

 been most kindly investigated by Mr. Grenville Cole, who himself 

 collected the different specimens he describes. 



Note on the Gravel of Ardtun. Bv Grenville A. J. Cole, Esq., 



F.G.S. 



The main constituents of the gravel-beds are flints and lava- 

 fragments, the proportions in which they occur varying considerably 

 in different layers. The larger masses are well rolled, the smaller 

 more so than would appear on fractured surfaces : and the features 

 of the beds are quite distinct from those of a tuff or a volcanic 

 breccia. The flints, despite their characteristically white and altered 

 condition, retain abundant traces of organisms and of the chalk from 

 which they have been derived. It may be fairly questioned, indeed, 

 whether these hard white fragments are not in many instances 

 comparable to the silicified chalk of the area rather than to the flints 

 developed by concretion in that chalk ; and whether they were not 



* The second leaf-bed of the Duke of Argyll, the third being unrepresented on 

 this side of the ravine. All these layers proved, however, to be so crumbling 

 as scarcely to repay working, and large quantities are left exposed in sific. 



t A very similar shale, with some of the same plants, was discovered in the 

 Isle of Canna by Prof. J. A. Harvie-Brown, who informs me that it occui's in a 

 cave on the north side of Canna, close to the shore. He says : — " Those we ob- 

 tained were at the exposed side of the shale-seam which projected from the rock 

 close to the floor of the cave. I think a fisherman, Mr. Isaac, if still at Canna, 

 could point out the place." 



+ Quart. Journ. Geol. Soc. vol. vii. p. 103, pi. i Berchcmia of Heer. 



