180 PLIOCENE AND MIOCENE FORMATIONS [Cii. XIV 



and another columnar bed of tlie same rock 10 feet thick is exposed at 

 the bottom of the cliff. One of the leaf-beds consists of a compressed 

 mass of leaves unaccompanied by any stems, as if they had been blown 

 into a marsh where a species of Equisetum grew, of which the remains 

 are plentifully imbedded in clay. 



It is supposed by the Duke of Argyle that this formation was accumu- 

 lated in a shallow lake or marsh in the neighborhood of a volcano, which 

 emitted showers of ashes and streams of lava. The tufaceous envelope of 

 the fossils may have fallen into the lake from the air as volcanic dust, 

 or have been washed down into it as mud from the adjoining land. The 

 deposit is decidedly newer than the chalk, for chalk flints containing cre- 

 taceous fossils were detected by the Duke in the principal mass of vol- 

 canic ashes or tuff.* 



The leaves belong to species, and sometimes even to famiHes, no longer 

 indigenous in the British Isles ; and " their climatal aspect," says Pro- 

 fessor E. Forbes, " is more mid-European than that of the English Eocene 

 Flora. They also resemble some of the Miocene plants of Croatia de- 

 scribed by Unger." Some of them appear to belong to a coniferous tree, 

 possibly a yew [Taxus) ; others, still more abundant, to a plane (Platanus)^ 

 having the same outline and veining well preserved. No accompanying 

 fossil shells have been met with, and there seems therefore the same un-. 

 certainty in determining whether these beds are Upper Eocene or Mio- 

 cene, which we experience when we endeavor to fix the age of many con- 

 tinental Brown-Coal formations, those of Croatia not excepted. 



These interesting discoveries in Mull naturally raise the question, 

 whether the basalt of Antrim in Ireland, and of the celebrated Giant's 

 Causeway, may not be of the same age. For in Antrim the basalt over- 

 lies the chalk, and the upper mass of it covers everywhere a bed of lignite 

 and charcoal, in which wood, with the fibre well preserved, and evidently 

 dicotyledonous, is preserved.f The general dearth of strata in the British 

 Isles, intermediate in age between the formation of the Eocene and Pho- 

 cene periods, may arise, says Professor Forbes, from the extent of dry land 

 which prevailed in the last interval of time alluded to. If land predomi- 

 nated, the only monuments we are likly ever to find of Miocene date are 

 those of lacustrine and volcanic origin, such as these Ardtun beds in 

 Mull, or the lignites and associated basalts in Antrim. On the flanks of 

 Mont Dor, in Auvergne, I have seen leaf-beds among the ancient volcanic 

 tuff's which I have always supposed to be of Miocene date. Some of the 

 Brown-Coal deposits of Germany are believed to be Miocene ; others, as 

 will be seen in the next chapter, are Eocene, Upper or Middle. 



Older Pliocene and Miocene formations in the United States. — Be- 

 tween the Alleghany mountains, formed of older rocks, and the Atlantic, 

 there intervenes, in the United States, a low region occupied principally 

 by beds of marl, clay, and sand, consisting of the cretaceous and tertiary 

 formations, and chiefly of the latter. The general elevation of this plain 



* Quart. Geol. Journ. 1851, p. 90. f Duke of Argyle, ibid. p. 101. 



