BRITISH ASSOCIATION Mli]ETING. 



27 



during the opening of the cavern ; but, on looking narrowly at the beds of 

 limestone in the progress of the workings, it was found that a thin seam of 

 pui'ple calcareous " slate" was interposed between the beds of limestone, at 

 about the same parallel as that in which the caverns were met with. On fur- 

 ther investigation, it was discovered that alternations of this purple " slate" 

 with the limestone were not unfrequent, but the laminae of slate were, in most 

 cases, so intimately blended with the limestone-beds, as to form really a solid 

 mass of compact rock ; and on looking into the structure of the more evident 

 layers of the " slate," it was ascertained that in some parts they were much 

 more calcareous than in others, and that small portions of limestone, having 

 similar physical characters to those of the surroundiag rock, were interspersed 

 at varying intervals. In other places the layers were in a state of decomposi- 

 tion, red and reddish white clay being formed as its result ; and on tracing a 

 layer of this kind through the side of a cavern laid open during the workings, 

 it was seen that portions of it were so disintegrated as to be easily pulled from 

 their position, the seam being, in its most solid portions, composed merely of 

 layers of limestone-fragments with interposed clay and red sand — ^the whole, 

 apparently, kept in place by the accidental infiltration of calcareous matter. 

 Here, then, were facts that might enable me to account for the clay found in 

 the caverns, and afford a means tlirough which the beds of limestone may have 

 been caused to separate from each other. Again, it was discovered that some 

 of the hoUows in the adjoining limestone were stained with a black earthy sub- 

 stance, found, on analysis, to be composed of the peroxides of iron and man- 

 ganese, these having evidently proceeded from the decomposition of a variety 

 of dolomite very generally present in this limestone — not exhibiting, however, 

 any definite mode of deposit in it, but passing through its beds in tlie most 

 irregular manner. Trom these phenomena, it appeared reasonable to conclude 

 that the decomposition of the " slate" in the layers, through the combined 

 agency of water and carbonic acid, had opened a communication with the 

 external air to the above-named irregular masses of dolomite (the unchanged 

 Limestone-fragments of the " slate" serving to keep the beds from close contact 

 with each other), and that in this way the carbonates of iron and manganese 

 contained in them liad been converted into peroxides, and the evolved carbonic 

 acid proceeding from their decomposition, combining with the remaining consti- 

 tuents of the dolomite, had formed bicarbonates, readily removeable by the 

 agency of percolating water. In this way it is possible, not merely to account 

 for the formation of the caverns, and a means of access to them, but at the 

 same time to discover what are the causes still in operation which give rise to 

 the production of stalactite, and occasion the irregular dolomization of the lime- 

 stone, it being evident that the percolating waters, charged with bicarbonates 

 of lime, magnesia, &c., may, by a loss of carbonic acid, deposit insoluble car- 

 bonate of lime in the form of stalactite, and becoming by this means richer in 

 bicarbonate of magnesia, act chemically on the neighbouring limestone, convert- 

 ing it into dolomite. 



To test the correctness of these views, a very careful examination of the clay 

 below the bones was iostituted : it was extremely tenacious, and of a dark 

 reddish-brown colour ; patches of red clay were visible in some places, and in 

 other parts of the mass distuict yeUow and black layers were apparent, and 

 nodules, or, more strictly speaking, irregular masses of impure ochry red iron- 

 ore, together with black, rounded fragments, evidently arising from the decom- 

 position of a dolomite similar to that before alluded to — for in the larger frag- 

 ments this rock was distinctly visible on fracture, and in one or two instances, 

 in which the masses were larger than usual, a brown zone was observable 

 between the black external coating and the central nearly unaltered dolomite ; 

 large and small masses of the common limestone-rock of the quarry were also 



