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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jail. 8, 



immediately under a sheet of basalt (seeMap, p. 90, and Fig. 2) . On the 

 Loch Scridden side this basalt is regularly columnar, some 20 feet high ; 

 on the Loch Laigh side it is much thinner, and somewhat resembles 

 a rotten trap. It is impossible not to conjecture from the line of dip 

 of the Ardtun strata, that this coal-seam is a prolongation of one or 

 other of its leaf-beds ; and, if so, it would be an interesting instance 

 of the passage of vegetable matter from one condition to another ; — 

 from a state, in which it is so slightly altered that every fibre of its 

 original structure remains, to one, in which it is converted into the 

 highly altered mineral, coal. 



To return to the strata exhibited at Ardtun. The geological 

 epoch, to which all the beds above that of the amorphous basalt 

 belong, is determined by the character of the organic remains. The 

 leaves are of considerable variety, but all belonging to well-known 

 existing families of the Dicotyledonous order. They are therefore 

 remains of the tertiary period ; — a conclusion farther confirmed by 

 the position of the chalk flints in the tuff conglomerates with which 

 they are associated. 



Further, these beds seem to me to furnish indisputable evidence of 

 subaerial volcanic action, alternating with periods of repose. The 

 second leaf-bed is the one which throws the clearest light on the cir- 

 cumstances of its formation. It is to be observed, in the first place, 

 that the leaves are not torn or shattered ; those of the large palmated 

 planes, as well as those of the small buckthorn, &c, being fully ex- 

 tended, and showing unruffled surfaces. Leaves, violently cast from 

 the trees on which they grew, would not have presented such appear- 

 ances. They do not even consist with the brittleness of dead leaves, 

 when dry. Two other remarkable circumstances remain to be 

 noticed ; first, that no trunks of trees, no branches, nothing beyond 



of the various beds at Ardtun. At a point east of the ravine where the lower 

 basalts are more easily measured, the whole series stands thus : — 



Feet. 



Uppermost basalt 40 



First leaf-bed 2 



First ash-bed 20 



Second leaf-bed 2£ 



Second ash-bed 7 



Third leaf-bed l| 



Amorphous basalt 48 



Columnar basalt to the level of low tide.. 10 



Total, 131 



At the ravine the measurement of the upper series of beds varies considerably from 

 those figures : — 



Feet. 



Uppermost basalt 16 



First leaf-bed 2 



First ash-bed 8 



Second leaf-bed 2\ 



Second ash-bed 6 



Third leaf-bed \\ 



From this it will be seen that whilst the leaf-beds preserve a remarkable uniformity 

 of thickness, the associated ash-beds and basalts vary much. In the uppermost 

 ash-bed there is a difference of 12 feet, and this within a short distance. 



