28 



THE GEOLOGIST. 



found in the clay, their surface being honey-combed as if by exposure to the 

 long-continued action of carbonated waters. These phenomena may justly be 

 explained on the supposition, that the irregular masses of ochry iron-ore had 

 been derived from the decomposed slaty seams, confirmatory appearances being 

 not unfrequent 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 admixture with it of the red oxide of iron 

 from the slaty seams, and the black oxide of manganese, accompanied by yellow 

 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 aU appeared of an opposing 

 character ; the clay was diligently 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 required for the dolomization of adjacent rocks : and in the hope 

 that a knowledge of such original cause might throw 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 connected 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 w^as deposited in the bone- 

 caves during the early period of their formation, and a portion, if not the 

 whole, of the time during which the bones were being introduced. My reasons, 

 . confirmed by the observations of Mr. Whidby, before alluded to, ai-e 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 di-y 

 habitation, and one in which the constant dropping of percolating waters would 

 give them no inconvenience, not to mention the constant disengagement of car- 

 bonic acid accompanying the deposition of the stalactite, which might even, 

 under some circumstances, render such caverns uninhabitable. 



In giving an opinion that the bones were introduced by animal agency, and 

 not by accidental falling into fissures, it is not to be inferred that, in no former 

 recorded instance, has this mode of entombment occurred. I will, 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. 



