274 



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



less defined, and all attempts have failed to trace their connexion 

 with the older, and perhaps also newer, flows of lava that proceeded 

 from that region. They show no decided dip either towards or away 

 from it ; and it is difficult to say where the terraced structure of the 

 plateau Traps definitely ends. That even then there was an ele- 

 vated tract not far off, possibly the Benmore region, which had been 

 the scene of acid eruptions, is indicated by the composition of the 

 Ardtun gravels, as described by Mr. Cole. But the crest of Benmore 

 itself appears to me to be a fragment of the wall of a crater which 

 may have occupied the site of Loch Beg, from which later lavas have 

 flowed *. It is an open question whether the traps ever actually 

 extended over the central part, or far beyond their present limits in 

 Mull, or over other districts where there are now no traces of them. 

 Contemporary dykes have, it is true, been traced by Geikie, Jack, 

 and others right across Scotland and even England, but these were 

 probably subordinate to the main fissures of eruption, which must 

 have been parallel to the long axis of the formation, and their 

 lavas need not necessarily have reached the surface. 



To return to Ardtun. We have already seen the horizontal extent 

 of the sedimentary series, and pointed out its position relatively to the 

 Traps which have enclosed it. It consists, where thickest, towards 

 the centre of the headland, of shaly clays and limestones, and coarse 

 indurated gravels and sands, which thin rapidly to the west and pass 

 beneath the sea, in not greatly diminished thickness, to the east. 

 The section given (fig. 1) shows them to be thickest near a ravine, and 

 again some distance eastward ; but in the latter locality the laminated 

 leaf-beds beneath the gravel, if present, are entirely concealed by talus. 

 The fossils have been obtained from the sides of the ravine, rendered 

 famous by the paper on them already referred to f , and more recently 

 from a spot a little to the east. 



On the west side of this ravine (fig. 2) we see, first, a bed of buff or 

 cream-coloured, soft, laminated sandstone (7i)+, the edges of which are 

 suddenly turned up and plastered in one place against the overlying 

 beds, suggesting that the ravine must be due to the upward passage 

 of a dyke which has since decomposed and weathered away — a view 

 supported by the gravel walls on both sides of the ravine, which have 

 the appearance of having been subjected to a more intense heat than 

 elsewhere. There are also some included fragments of a pale drab- 

 coloured stone on the opposite side (fig. 3). On the east side of 

 the ravine I undertook, by aid of a Government grant from the Royal 

 Society, some rather extensive quarrying operations, and the follow- 

 ing beds were seen : — At the base, on the amorphous Trap (?), is car- 

 bonaceous rubble filling in its rugged surface, which may be set down 

 at 1 foot. Above this is 2 feet of bedded river-sand (g), now indurated, 



* The structure of the peak resembles that of the peaks forming the walls 

 of the Grand Curral in Madeira, though denuded to an infinitely greater 

 extent. 



t Argyll, Quart. Journ. Geol. Soc. vol. vii. p. 89. 



+ This occupies the position of the third leaf-bed of the Duke. 



