258 Proceedings of the Royal Irish Academy. 



calcite or metallic veins. The individual beds are all of considerable 

 thickness, the thinnest one noted being not less than 4 feet. 



The caves are situate on the northern side of the valley, and, as might 

 be expected, the strata at this point dip south; the dip was carefully 

 observed at all points where it was clearly visible, and has been noted in the 

 plans ; it ranges from 30° to 40°.^ It is owing to this dip (see Plate XVI., 

 fig. 2) that the formation of these caves exhibits so many features of interest, 

 such as do not exist in the caves of Yorkshire, Derbyshire, and Fermanagh, 

 where the stratification is practically horizontal, although in the Great 

 Eastwater Cave on the Mendips there are chambers similar to the great 

 chambers of the Old Cave, but on a less impressive scale. 



The types of passages and chambers may be divided into three main 

 groups, depending for their characteristics in a great measure upon the 

 direction of their greatest length in relation to the dip of the strata ; in fact, 

 the direction of any passage, with a few exceptions, can be determined by 

 an explorer from a consideration of its type. 



In all limestone caves the chief passages and chambers are formed in a 

 great measure by the action of water upon the various planes of weakness in 

 the rock. These planes are usually three, or occasionally four, in number. The 

 first consists of the plane of stratification, and the others of joints running at 

 right angles to this. In certain cases these joints run through only one bed 

 of the limestone, but in others cut through many beds; and, in fact, seem to 

 partake more of the nature of faults than of mere joints. In certain of these 

 latter cases in caves elsewhere^ slickensides have been observed, thus showing 

 that the chamber or passage is the direct result of faulting, and is thus really 

 an open fault. 



In the Mitchelstown Caves there are, so far as could be ascertained, only 

 three planes of weakness which have contributed to the formation of the 

 passages — 1. The bedding planes (dipping south). 2. The main joints (running 

 north and south, and cutting in a continuous line vertically through many 

 beds). 3. Secondary joints (running east and west, and cutting through only 

 one bed at a time). Each of these gives rise to a plane of weakness through 

 which water can percolate. This aqueous percolation (containing carbon 

 dioxide in solution) slowly dissolves the hard limestone on either side, and 

 ultimately forms a space through which running water can find a way. This 

 increased flow, carrying with it sand and stones, has a mechanical as well as a 



> The Geological Survey Map marks the stratification as horizontal ; this is a cartographical 

 error. 



^ Yorkshire Ramhlers' Cluh Journal, vol. ii.. No. 6, pp. 157-159. 



