52 Prof. Percy F. Kendall — On ' Cleat ' in Coal-seams. 



followed by more shales with a couple of small coal-seams, but near 

 the top of the picture can be seen the rectangular salients and 

 re-entrants of a boldly jointed bed of hard sandstone. "While the 

 shales are shattered by irregular joints in all directions and in various 

 azimuths, and therefore are valueless for comparison with the 

 cleat, the sandstones afford trustworthy data. The photograph 

 shows quite clearly that the prolongation of the cleat would almost 

 exactly bisect these joints (see Figure). The case is quite typical of 

 the unrelated disposition of the cleat to the jointing of the measures. 

 I refrain for the present from entering into any discussion of the 

 cause of the cleat, but there is one question that seems to admit of 

 an answer or at least a suggestion even at this early stage of the 

 investigation, and I venture to deal with it now in the hope that 

 I may elicit from some reader with a knowledge of lignite beds 

 abroad information of which I am much in need. 



The question I refer to is — Why does the jointing in the coal take 

 a course absolutely unrelated to that of the enclosing measures? The 

 first proposition I would advance is the obvious one that the two sets 

 were produced by forces operating in different directions and at 

 different times. The cleat would, I imagine, be produced first — 

 otherwise it is difficult to understand why a fragile substance like 

 coal should have escaped shattering by the force that jointed the 

 other rocks, whereas if it had already acquired a cleat it might yield 

 to later strains or stresses without the production of a fresh system of 

 fractures. 



Pursuing this inquiry further, we should ask to what property 

 coal, or the peaty mass out of which the coal was later formed, owed 



