﻿98 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 8, 



These appearances, however, are hardly sufficient to determine 

 whether the tuff was erupted in the form of a muddy flood, or in that 

 of a volcanic shower. A fall of any height through air would, to a 

 certain extent, have the same effect as subsidence through water, in 

 producing an arrangement of particles, determined by their specific 

 gravity. Subsequent heavy falls of rain, however, such as have fre- 

 quently been known to accompany similar eruptions in the present 

 epoch, by washing down the finer particles, might considerably 

 modify the original conditions. It must be remembered, however, 

 that in the disposition of matter erupted by a volcano, much must 

 depend on the degree of proximity to the seat of action. The nearer 

 that point is, the more completely will the laws of gravity be liable 

 to be counteracted by more violent temporary forces. Now, I think 

 there is some evidence, that the seat of action was at no great distance 

 from the spot described. The tuff-bed is traceable for some distance 

 along the coast ; and at one point, the farthest at which I have ex- 

 amined it, the composition is fine-grained and homogeneous, indicating 

 that at the spot referred to, the eruption had undergone a change 

 similar to that which distinguished the fine white ashes which covered 

 the country at Misenum, from the coarser and mixed materials which 

 overwhelmed Pompeii. I have already described also the remarkable 

 change which takes place in the composition of the bed within a few 

 feet of space at the Ardtun ravine. So rapid a transition in charac- 

 ter, from a conglomerate of coarse and heavy materials to one of much 

 finer composition, seems to indicate a corresponding rapid change in 

 the intensity of the forces to which the bed owes its origin. The 

 condition of the flints tends to prove the agency of heat ; whilst it 

 equally proves that the degree of intensity to which they were ex- 

 posed was very variable. Some of them have the appearance of 

 having been much burnt, although I observed none in a state of vitri- 

 fication ; whilst others are so little altered as to preserve in good 

 form minute organic remains. From these circumstances, although 

 on this point sufficient data are still wanting, I should be inclined to 

 conclude that the flints had not been thrown out in a fiery shower, 

 but rather having been subjected to considerable heat, modified by 

 the earthy matter with which they were associated, were poured forth 

 with it in a mud-stream. But, whatever may have been the particu- 

 lar process by which the tuff-beds, and this one especially, may have 

 been formed, it is certain that it must have been repeated after a con- 

 siderable interval of time, and that the volcanic eruption was not of 

 such violence as to change materially the conditions of the surface. 

 The hollow in which the marsh had originally been formed, and in 

 which the first or lowest leaf-bed had accumulated, continued to be a 

 hollow after the mud and ashes had overflowed it. Water again ac- 

 cumulated, and autumnal leaves were again cast upon its surface in 

 greater numbers and variety than before. An eruption similar to 

 the first for a second time covered its deposits ; still its condition re- 

 mained sufficiently unchanged to admit a repetition of the same pro- 

 cess, and once more it continued to receive the annual sheddings of a 

 forest vegetation. But the third eruption must have been one of a 



