148 The Giant's Cave. 



or refraction, when the two media are of equal refractive densities ; and 

 that these effects are less as the refracting energies of media approacli 

 nearer to equality. This bears hard on the Newtonian doctrines ; how- 

 ever, the two last articles and the fifth will be sufficient for the reader to 

 form an estimate ; we will tlierefore proceed to explain the same facts, on 

 the hypothesis of undulations. 



(To be Continued.) 



THE GIANTS CAVE. 



Among the fissures and cavities which we propose from time to time 

 to describe, as occurring in the carboniferous limestone of the West of 

 England and Wales, are a considerable number, to which, from their inac- 

 cessibility, or from the circumstance of their being supposed to contain no 

 bones, but little attention has been paid, or indeed supposed to be due. 



Independently, however, of the interest which attaches itself to bone 

 caves, or to fissures remarkable for their magnitude, or tlie splendour of 

 their stalactitical or stalagmitical encrustations, the consideration of rock 

 cavities and clefts seems to promise some valuable points of inference, and 

 may not unreasonably be expected to throw a certain portion of light upon 

 some of the obscure parts of theoretical Geology. 



Our readers are probably well aware, that concerning the age and man- 

 ner of formation of those clefts and cavities, geologists are by no means 

 agreed, and are probably more ignorant upon this than upon any other 

 branch of tlieir science. 



The only method by which the elucidation of these subjects can reason- 

 ably be expected, must be by a careful examination of a vast number of 

 instances, remarking the figure, dimension, and direction of each, the kind 

 of surface presented by its interior, the nature of its communication with 

 other cavities, its contents, and the condition of the rock in which each 

 may occur. Attention to tliese circumstances, even if it give rise to no 

 positive results, may yet be expected to disprove several hypotheses upon 

 these subjects J and thus by this negative kind of proof, reduce the real! 

 question within narrower limits. 



The cave, with a description of which it is our intention to occupy the 

 present paper, is one well known to all the inhabitants of Bristol and its 

 vicinity, although froni the difficulty and danger — almost amounting to im- 

 practicability — of the access to it, it has hitherto, as far as we are aware, 

 remained undescribed by either geologist or antiquary. 



The Giant's Cave, so called from some fabulous tradition, is contained 

 within the upper beds of the carboniferous limestone, these dipping south- 

 east at 23°, and forming St. Vincent's Rock. The cavern opens upon the 

 precipitous escarpment of the rock, at a height of about two hundred and 

 fifty feet above the river, and fifty or sixty below and to the west of the 

 Observatory. A rude and broken ledge extends from the north-eastern 



