28 
THE GEOLOGIST. 
found ill the clay, tlieir siirfaec being lioncy-combcd as if by exposure to tlie 
long-continued action of carbonated waters. These phenomena may justly be 
explained on the supposition, that the irregular masses of ocliry iron-ore bad 
been derived from the decomposed slaty seams, confirmatory appearances being 
not uufreqiient in other limestone-beds connected with the same series of rocks, 
the "slate" in these alternating with the limestone on a large scale, and con- 
taining irregular nodules of impure iron-ore— a red oxide of iron being fre- 
quently visible at the points of junction. The varied colour of the clay may 
also be accounted for by the gradual admixtiu-e with it of the red oxide of iron 
from the slaty seams, and the black oxide of manganese, accompanied by yeUow 
hydrated peroxide of iron from the dolomitic rock, which may be concluded to 
have formed a part only of the walls of the cavern — the honey-combed lime- 
stone fragments resulting from the displacement of other portions of previously 
fissured limestone-rock through the agency of aqueous carbonic acid. The 
most careful examination presented no facts that at all appeared of an opposing 
character ; the clay was culigently searched, and some of its laminated portions, 
having a sandy appearance, were examined by the microscope for the siliceous 
coverings of infusoria, minute rounded grains of sand, and any other matter 
that might suggest the washing in of the contents of the cavern through free 
communication of its opening with external waters ; nothing was, however, 
discovered but very minute fragments of slate, still further confirmatory of the 
position before advanced. 
The facts elicited were thus far satisfactory, but they did not account for 
the original production of those masses of dolomite, which in the neighbour- 
hood of the quarry, alone afforded, by their own decomposition, the solution of 
bicarbonates requked for the dolomization of adjacent rocks : and in the hope 
that a knowledge of such original cause might tlirow still further light upon 
the present condition of the bone-caves, a general examination of the various 
accessible quarries of the Plymouth limestones was instituted. 
I propose to give some account of these investigations in the latter part of 
this paper ; and will now proceed to give a brief description of the fossil re- 
mains, and certain circumstances eomieeted with them, as the following out of 
the inquiries alluded to will lead me to speak, not merely of changes having 
an important relation to the phenomena of the enclosed caverns, but also to 
the attempted solution of other allied geological questions of interest. 
I am disposed to believe that very little stalactite was deposited in the bone- 
caves during the early period of their formation, and a portion, if not the 
whole, of the time during wliich the bones were being introduced. My reasons, 
confirmed by the observations of Mr. Whidby, before alluded to, are the fol- 
lowing : — The bones have been generally found lying on or near the uppermost 
portion of a bed of clay, and those on its surface only are much mixed with, or 
imbedded in stalagmite, the remains met with lowest in the clay being especially 
free from such deposit. It is reasonable also to suppose that, if the fossil 
bones were introduced through the agency of carnivorous cave-inhabiting mam- 
malia, the instincts of these creatures would have induced them to prefer a dry 
habitation, and one in which the constant di-opping of percolating waters would 
five them no inconvenience, not to mention the constant disengagement of car- 
onie acid accompanying the deposition of the stalactite, which might even, 
under some cii'eumstances, render such caverns uninhabitable. 
In giving an opinion that the bones were introduced by animal agency, and 
not by accidental faUing into fissures, it is not to be inferred that, in no former 
recorded instance, has this mode of entombment occui-red. I wUl, however, 
give some facts connected with the nature and mode of occurrence of these re- 
mains, before attempting to deduce any further conclusions in the present 
instance. 
