THE UNDERGROUND WATERS OF NORTH-WEST YORKSHIRE. 27 
Now in all stratified rocks which are jointed it is found 
that of the two dominant sets of joints usually decussating at 
angles of 70° to 90° one set, the master-joints, is much stronger, 
i.e., is much more persistent both in continuity and direction 
than the other set, which may be called for present purposes 
" cross-joints." 
It is safe to assume that the master-joints from their greater 
continuity will afford a freer flow to water, and this will secure 
that they will be proportionately more affected by the solvent 
action of water, so that, in any district, unless the direction of 
easiest ultimate escajDe for underground water be directly in the 
line of the cross-joints, it will travel as far as possible along 
the master-joints. 
Now to apply these principles to the problem of the sources 
of the Aire. The stud}^ of the six-inch Ordnance map showed 
that the high limestone plateau, beneath which the subterannean 
waters flow between the several sinks and springs, is traversed 
by lines of scars having a very marked jDarallelism ; some of 
the valleys, moreover, or portions of them run in straight courses 
with the same general orientation ; and it was suggested that 
these features were determined by the master-joints, of which no 
field observations had, up to that time, been made. 
Certain obvious reflections were also made upon the results 
already obtained by the Chemists, which lent an air of proba- 
bility to the hypothesis that the features in question were due 
to master-joints which had also a preponderating effect upon 
movements of the underground water. Thus a line drawn from 
Smelt Mill Sink to Malham Cove coincided with the general 
direction of the scars and other features of the plateau, wdiich 
were assumed to depend upon the master-joints ; it was also 
almost coincident with the long valley of the Watlowes and 
a train of intermittent valleys and scars running nine-tenths of 
the distance ; but, further, it joined together the actual points 
of entry and emergence of water which had been traced by 
the Chemists. 
