EXCURSION VI. 125 



summer: The strata are exposed from a few feet outside 

 high water mark to the limit of low water, whence they 

 dip under the sea; the horizontal extent is about 150 yards; 

 the dip is K 10 E. at 39 to 60. There are eleven distinct 

 beds of shales, trappean ash, and thin coal seams. The 

 ash beds are two to three feet thick, and enclose the trees. 

 The trappean ash closely resembles certain indistinct earthy 

 greenstones, with a semi-crystalline structure, but is really 

 an aqueous deposit, distinctly stratified and made up of 

 materials derived from plutonic sources, re-arranged by 

 water, and afterwards indurated by heat and pressure; and 

 is wholly different from the ordinary trap rocks of Arrau, 

 which are of much later origin than any of these beds, as 

 they cut through strata of much later age. But there 

 has been igneous action at all periods; and this case of 

 itself is enough to assure us that it was vigorous in the 

 lower carboniferous period. There must have been in this 

 neighbourhood a volcanic vent which showered forth ashes, 

 by which the trees in their growing position were successively 

 enveloped; while the existence of these trees on different 

 horizons shews long-continued periods of repose and vegetable 

 growth, and successive sinkings after each; so that the 

 vegetation out of which the coal was formed had time to 

 accumulate. Such frequent repetition of the surface of 

 growth, with intervening subsidence, is shewn in the coal 

 formation everywhere; and there are cases, 4n other parts of 

 Scotland, of beds very similar to these, in the lower car- 

 boniferous series, though without the remains of the ancient 

 forests so skilfully exhumed by Mr. Wiinsch. The trees 

 plainly grew upon a horizontal surface, which got after- 

 wards tilted up into its present inclined position, either 

 by the intrusion of the traps in the vicinity, or by the 

 upheaval of the central granite, which, we have seen, 

 may have occurred after the lower carboniferous beds were 

 deposited. 



The beds are shewn on the annexed diagramatic section 

 (fig. 23) in the position, and of the terraced form, into which 



