﻿1851.] 



DUKE OF ARGYLL ON ARDTUN LEAF-BEDS. 



.97 



the size of the merest twig, has been yet found associated with the 

 leaves ; secondly, that plants of a reedy texture — some of them at 

 once recognisable as Equiseta — are associated in great abundance with 

 the leaves, especially with that lower portion of the bed which almost 

 exclusively consists of the vegetable remains. From all this the con- 

 clusion is obvious, that these leaves must have been shed, autumn 

 after autumn, into the smooth still waters of some shallow lake, on 

 whose muddy bottom they were accumulated, one above the other, 

 fully expanded and at perfect rest. It cannot have been a water agi- 

 tated by tides or currents ; for these would have swept such remains 

 away, or left evidence in their disposition of disturbing agency. It 

 cannot have been water of any depth ; for it is well known that reeds, 

 and especially the Equisetum and other kindred families, do not affect 

 such situations. But there is another ground for this latter conclu- 

 sion : the bed of ashes or tuff covering the leaves shows clearly, from 

 the arrangement of its materials, that they cannot have undergone 

 the sifting process inseparable from subsidence through water. The 

 light pumiceous particles and the heavy flinty white lapilli, are dis- 

 seminated indiscriminately without any reference to the order of 

 gravity, although the former are composed of a substance which will 

 frequently float in water, whilst the latter are particularly dense and 

 heavy. 



All these circumstances taken together, as also the absence of any 

 freshwater shells, or other organisms, indicative of a permanent lacus- 

 trine condition, seem to me to afford the strongest evidence, that the 

 situation in which these leaves were overflowed by volcanic mud and 

 ashes, was one, which may rather be described as a marshy terrestrial 

 surface, than the bottom of a lake, properly so called. If this con- 

 clusion be correct, it follows that the materials which overlie the 

 leaves were emitted by a volcano in subaerial action. The condition 

 of this matter at the time of its eruption seems pretty clearly indi- 

 cated by its condition now. First, the damp and bedded leaves have 

 had poured upon them a stream of liquid mud, insinuating itself be- 

 tween their planes, lifting and holding those most easily detached from 

 the surface, and leaving in its original state of rest the lower portion 

 of the bed. To this matter, — which although now appearing in its 

 upper portion as a hard blue stone, bears in the perfect state of its 

 vegetable impressions indisputable evidence of its once liquid condi- 

 tion, — has succeeded an overflow or a shower of matter of very dif- 

 ferent composition. In respect to the latter, it is more difficult to 

 conclude with certainty what was the original condition. It seems to 

 have followed the mud after a very short interval of time, although 

 long enough to have allowed a partial consolidation. That stray twigs 

 and leaf-stalks were still sticking out of the surface of the mud, is 

 sufficiently proved by their traces, generally much carbonized, in the 

 lower part of the tuff. The line of junction between the bottom of 

 the tuff and the top of the leaf-bed is, in a general view, sharp and 

 definite enough ; whilst a closer inspection shows just enough subsi- 

 dence of the particles of ashes into the substance of the stone below, 

 to indicate the degree of consolidation to which the latter had attained. 



