1865.] BRITISH ASSOCIATION. 447 



Since the large masses of limestone occur at all levels in the 

 cave earth, as well as everywhere above it, it is obvious that what- 

 ever may be the cause to which their fall is attributable, they can- 

 not be referred to any one and the same period. They fell from 

 time to time throughout the accumulation of the cave earth ; they 

 continued to fall whilat the stalagmitic breccia was in process of 

 formation, as well as during the introduction of the black mould, 

 and they are amongst the most recent phenomena which the cavern 

 presents. And even of those which lie on the surface, there is a 

 conclusive evidence that in some cases a considerable length of 

 time must have elapsed between the fall of two blocks, one 

 on the other; an interval sufficiently great for the formation for 

 the cake of stalagmite between them, and which is sometimes full 

 six inches thick. There can be little doubt that some of them fell 

 very recently even when measured by human standards. 



It is by no means easy to determine the cause which threw 

 them down. To call in the aid of convulsion seems undesirable, 

 since it would be necessary to do so very frequently. Moreover it 

 may be doubted whether anything short of a violent earthquake 

 would be equal to the effect. Though the roof of the chamber is 

 of very great span and entirely unsupported, and though it 

 presents appearances which are not calculated to inspire confidence, 

 the violent concussions produced by the frequent blastings already 

 mentioned, blastings which not unfrequently throw masses of lime- 

 stone, weighing upwards of a ton, to a distance of several feet, 

 have never brought down even a splinter. 



The fall of the blocks has sometimes been attributed to 

 changes of dimensions in the roof, arising from changes of tem- 

 perature, but the fact that the cavern temperature is all but con- 

 stant throughout the year seems fatal to this supposition. 



The masses lying on the surface were a sufficient guarantee 

 that the deposits beneath them remained intact. There can be no 

 doubt that they are at once a proof, and the cause of the undis- 

 turbed character of the soil they cover. A portion of the cavern 

 so easily accessible as is this chamber would not have been spared 

 by Mr. McEnery but on account of some great difficulty or dis- 

 couragement; and in fact he states that the fallen masses com- 

 pletely foiled him in his attempts to make explorations in it 7 

 excepting in one branch some distance south of the area selected 

 by the committee. Their own characters, moreover, render it 

 absolutely certain that the deposits have never been violated. 



