December 16, 1922] 



NA TURE 



81 



that it can be brought within the title of this address 

 — I allude to the cleavage or cleat or slynes of coal. 

 If we look at a piece of coal this cleavage is very 

 conspicuous, for. lying at right angles with the bedding, 

 it gives the straight sides to the fragment. It is 

 obviously not like the cleavage of slate, a texture, but 

 it is a series of well-developed joints. 



It is a vital element in the cleat problem that it 

 is as well developed and as definite in direction in 

 a flake of bright coal the j^th of an inch in thickness 

 as in a tree-trunk. While I was preparing this address 

 I procured a slab of shale from the bed underlying 

 the uppermost bed of the Millstone Grit. It bore 

 numerous imprints of goniatites and a leaf of Cordaites. 

 which, in its present condition of bright coal, varies 

 in thickness from about J^th down to yi^th of an inch 

 in thickness. It is traversed by an even and regular 

 cleat at intervals of about , J^th of an inch, disposed 

 at an angle of about ^ to the length of the leaf. 

 With great care it was possible to replace the slab 

 in its original position and to determine the orienta- 

 tion of the cleat to be N.W.-S.E. This is not nearly 

 the extreme of tenuity reached by well-cleated plant 

 remains. I have specimens that are mere shiny films, 

 and cannot, I should judge, exceed T jj„th of an inch, 

 yet they show well-defined and regular cleat. Further, 

 it should be noted that the production of cleat was 

 subsequent to the erosion of stream channels as well 

 as to the production of phenomena on the margins 

 of the wash-outs. Every pebble and flake of coal 

 found in the displaced masses in these stream-casts 

 has the cleat well developed, and in strict parallelism 

 with the cleat of the adjacent undisturbed seam. 



I have directed attention to the fact that cleat is 

 quite independent of the joints traversing the shales 

 and sandstones of the associated measures ; whence 

 I draw the inference that the cleat must have been 

 produced prior to the jointing. 



The reason for this early development of a joint 

 system is easily found — the original peat, in passing 

 into lignite, acquired a brittle consistency and a 

 consequent disposition to joint. Indeed, the change 

 of consistency is the effect of chemical change and 

 loss, whereby the peat substance contracts. Hence 

 when our Coal Measures w-ere first laid down they 

 would consist of a series of incoherent sands and muds, 

 and this uncompacted condition may have persisted 

 for a very long period, even surviving considerable 

 tectonic disturbances. The peats, however, would be 

 subject to changes entirely innate : the gradual loss 

 of volatile constituents, or at least the resolution of 

 the carbon compounds into new groupings and the 

 conversion of the mother substance of the coal into 

 lignite. In this condition the coal-substance would 

 be brittle and liable to joint in response to the tensile- 

 strains set up by the contractility of the mass. 



There are questions of very deep import concerned 

 with the geographical 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 

 Ashby-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 



NO. 2772, VOL. I IO] 



is a subject which hitherto has not been much con- 

 sidered. 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 direction 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." 



In a paper in the Geological Magazine I commented 

 on the fact that little had been written on the subject 

 of cleat since Jukes's " 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 corroborates the statement so far as 

 concerns the coalfields of Northumberland and Durham, 

 where he says it " runs most generally to the north- 

 west (true)." The same direction, he says, prevails 

 in Yorkshire and Derbyshire and also in Lancashire. 



I have suggested a reason why coal should acquire 

 a joint system anterior to, and independent of, that 

 of 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. 



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. 



Cleat observations in the Northern Hemisphere show 

 an overwhelming preponderance of a N.W.-S.E. 

 direction in coals and lignites of all ages from Carbon- 

 iferous to Pleistocene and from regions so remote as 

 Alaska, Spitsbergen, the Oxus, Nigeria, and China. 

 This direction persists through every variety of tectonic 

 relations, but seems most regular in the largest and 

 least disturbed fields. 



Jukes's miner's astonishing statement that " the slyne 

 faces two o'clock sun ... all over the world " involves 

 more than is at first glance apparent, for, as a friend 

 lias ] minted out, that two o'clock sun must shine from 

 a quite different compass-bearing in the Northern and 

 Southern Hemispheres. Yet the data I have collected 

 confirms generally the miner's declaration in the 

 Southern Hemisphere as well as the North, though 

 exceptions occur that may possess a deep significance. 



Many of the southern coals have no definite cleat, 

 but in such as do display a regular system there is a 

 distinct predominance of the N.E.-S.W. direction, 

 which has a curious inverse relationship with the 

 N.W.-S.E. direction of the Northern Hemisphere. 



I feel persuaded that the cause will be found in some 

 relation to influences, tidal or other, dependent upon 

 the earth's planetary role. 



There is a negative aspect of the "cleat question 

 which brings it more clearly within the ambit of an 

 inquiry into the physiography of the coal-swamps. 

 I allude to the absence of cleat that characterises 

 anthracite the world over. Upon this absence of 

 cleat are attendant features that have been regarded 

 as indicative of conditions prevailing during the 



