C— GEOLOGY. 78 



to joint in response to the tensile strains set up by the contractility of 

 tlie mass.* 



There are questions oi' very deep import concei'ned witii the geo- 

 graphical direction of the cleat. The first reference to this interesting 

 topic is, I believe, in a work, close upon a century old, by Edward 

 Mammatt, entitled Geological Facts to elucidate the Ashhy-de-la- 

 Zouch Coalfield, published in 1834. His fourth chapter, headed ' On 

 the polarity of the strata and the general law of their arrangement, ' 

 contains these remarkable passages : ' Polarity of the strata is a subject 

 which hitherto has not been much considered. The extraordinary 

 uniformity in the direction of the slynes and of the partings of the 

 rocky strata seems to have been determined by the operation of some 

 law not yet understood. . . . Wherever these slynes appear, their direc- 

 tion is 23° West of North by the compass, whatever way the stratum 

 may incline. The coal between them has an arrangement of lines all 

 parallel to the slynes, by which it may be divided. This is called the 

 end of the coal. . . . Many of the Derbyshire and Nottinghamshire 

 Coal Measures have their slynes in the same arrangement as the strata 

 upon Ashton Woulds, and this is also preserved in those of Ooleorton, 

 about five miles to the westward.' 



In my paper in the Geological Magazine I commented on the fact 

 that little had been written on the subject of cleat since Jukes' Manual 

 of Geology (1862), in which he quotes a Nottinghamshire miner's remark 

 that the slyne faced ' two o'clock sun, like as it does all over the world, 

 as ever I heered on,' a generalisation to be remembered. 



John Phillips, in a report presented to this Section in the year 1856, 

 corroborates the statement so far as concerns the coalfields of North- 

 umberland and Durham, where he says it ' runs most generally to the 

 north-west (true).' The same direction, he says, prevails in Yorkshire 

 and Dex'by shire and also in Lancashire. 



I have suggested a reason why coal should acquire a joint system 

 anterior to, and independent of, that ol the associated measures, but 

 while providing a jointing-force that theory furnishes no explanation of 

 the directional tendency of the cleat. This tendency must have been 

 supplied by some directive strain — not necessarily of great intensity, 

 but continuous in its operation. 



The idea that the initiation of joints, as it were the pulling of a 

 trigger, is due to seismic tremors, is urged by Mr. W. 0. Crosby, but 

 it seems that an agency much more constant in operation and direction 

 is required. 



In 1914 and since I have collected a great body of data regarding 

 the direction of the cleat in coals and lignites in many parts of the 

 world, chiefly by means of cii'cular-letters to every colliery manager 

 in the Britisli Isles and to many abroad. I have also obtained most 

 genei'ous help and information from valued correspondents in the United 

 States, foremost of whom I must mention Professor J. J. Stevenson. 



Cleat observations in the Northern Hemisphere show an overwhelm- 

 ing preponderance of a N.W.-S.E. direction in coals and lignites of all 



* Fusain. being already greatly decomposed, would not be as brittle and 

 would not cleat so readily. 



