HrLL, Brodrick, and Rule — The MitcheUtown Caves. 261 



subsequently the roof has collapsed, blocking up the old water-channel, and 

 thus forming a passage at a higher level. In such cases the floor of the 

 present passage exhibits no sign of water action, and frequently presents a 

 cast of the inequalities of the roof. There is direct evidence that a collapse 

 of this nature has occurred in a portion of O'Callaghan's Cave. After leaving 

 the Scotsman's Cave, one traverses a slightly falling water-tunnel, along 

 which a considerable stream has flowed at some time. At a certain point 

 this passage is partially obstructed for about 30 feet. This obstruction is 

 formed by a mass of the roof which has fallen at some period, and can now be 

 passed only by climbing over it. The passage above this block represents all 

 the characteristics of type No. 2. and thus indicates the method of its 

 formation. In the case referred to, the fallen mass has as its two upper faces 

 the bedding-plane and the secondary joint — two lines of weakness which 

 allowed the fall to occur. It is even possible that in certain cases this type 

 of passage has been formed as the result of erosion some distance away, which 

 might cause a local dislocation of the strata. The secondary joints in this 

 district run east and west, with a result that any passages of this type have 

 an easterly and westerly direction. These joints are at right-angles to the 

 plane of stratification, and do not seem to cut through more than one bed in 

 a continuous line, so that the height of the passage at any place depends upon 

 the thickness of the bed of limestone at that point. As far as was observed, 

 there was no passage or chamber whose existence could be attributed to the 

 secondary joints alone. But these joints are evidently very important 

 contributing causes to a large number of passages, especially in the New 

 Cave. 



Passages of Type 2. — Demon's Cave, parts of O'Callaghan's Cave ; 

 passages west of Cust's Cave, and east of Eabbit-hole ; Brog den's Cave, and 

 the Chamber east of the Labyrinth. 



The third, and in some respects the most important, line of weakness is to 

 be found in the main joints, or, as they are sometimes called, master joints, 

 These run north and south, and cut through the limestone in continuously 

 vertical lines, being apparently entirely unaffected by the bedding-planes. 

 They have given rise to two main types of passages. Firstly, those in which 

 subsequent erosive water action is not apparent ; and, secondly, those in 

 which it is. 



The first type consists of those narrow fissures of which the best examples 

 are to be found in the Maze. The fissures in this part of the New Cave are 

 eight in number ; and if we add to these the fissures known as the Closets, 

 which are entered from the Sand Cave, which, as will be seen from the plan, 



