228 REPORT— 1902. 



It was seen that in the open passages in the neighbourhood of Alum 

 Pot was an opportunity for putting this theory to the practical test by 

 following the water step by step in its underground journeyings. 



With this object in view your Committee undertook to make a sur- 

 vey of some of the more readily accessible channels, with the following 

 results. 



A plan of the series of passages communicating with Long Churn and 

 joining that opening with Alum Pot was constructed, and tins will be 

 published in due course by the Yorkshire Geological and Polytechnic 

 Society. 



On this plan the directions of the principal lines of jointing liave 

 been marked for purposes of comparison Avith the direction of the 

 caverns. 



It may be confidently said that there is a general parallelism between 

 joints and passages, but this is by no means so close as was at first 

 expected. 



To account for the want of parallelism between joints and passages 

 it is necessary to study the evolution of one of these underground 

 chambers. 



The joints in the limestone undoubtedly give the initial direction to 

 the underground stream ; but as soon as a channel is formed sufticiently 

 large to allow of a free flow of watei*, as opposed to mere soakage, a 

 number of other forces come into play which tend to modify the direction 

 so as to cause it to diverge somewhat from its original one of strict 

 parallelism to the joints. 



For example, the dip of the rocks causes the erosion of the channel 

 to be more severe on one side than the other — namely, on the low or 

 ' down dip ' side — and where there are cross-joints the dip may tend to 

 produce a lateral escape along one of these so as to give rise to a zigzag 

 course. 



Up to a certain point the erosion in these underground river channels 

 is entirely by solution, but so soon as the external opening becomes suf- 

 ficiently large to admit sand gravel and boulders excavation by means of 

 attrition comes into play. 



At this point in the history of a subterranean river zigzags are 

 changed into sharp windings, which in their turn impart a swing to the 

 waters in the straight parts, thus causing a series of windings to be set 

 up in a manner similar to that which goes on in surface streams. 



As the external opening which gives access to the water increases in 

 size, so does the amount of water flowing through the passage increase. 

 An increase of volume means an increase of speed and a lessened ten- 

 dency to winding ; at the same time the passage must be widened so as to 

 accommodate the increased volume of the stream. 



The tendency at this stage is, then, to widen and straighten out the 

 passages, and many are the deserted ' ox-bows,' both large and small, 

 which may be seen in the passages. 



Some of these are small and situated at a considerable height above 

 the floor of the main passage, while others of more recent formation 

 are approximately at the same level as the water-bearing passage, a 

 few of them still being occupied by a portion of the stream in times of 

 flood. 



The condition of approximate stability is reached when the opening 

 becomes sufliciently large to swallow the whole of the surface stream. 



