Correspondence — Mr. W. 8. Gresley. 523 



very colour of the clay (as reported) being different from the clays 

 of the immediate neighbourhood ; all pointed to this conclusion. 

 He therefore urged that this Chapelhall clay should no longer be 

 cited as a proof of submergence. 

 An interesting discussion followed. 



OOE-E-IESI^OlSrZDElSrCIE. 



THEORY FOR "CLEAT" IN COAL-SEAMS. 



Sir, — It must surely be admitted by students of the Coal-measures 

 in every country, where coal-mining is carried on to any extent, 

 that coal-seams are the most persistent in extent of area, most 

 uniform in composition and in homogenity of any strata of the 

 series ; and therfore may justly be considered the typical beds of it. 

 The master-joints or "cleat" of coal are much more regular than 

 those of any other strata; and I think Coal-beds were much more 

 likely to shrink and crack evenly than the less persistent and ever- 

 varying associated shales, clays, and sandstones. The least-disturbed 

 Coal -areas exhibit the best-developed or most regular and typical 

 "cleat." Thus, if "cleat "was formed or produced by shrinkage 

 of the mass in cooling, due to elevation following deep subsidence 

 (which I think is the generally accepted opinion as to what caused 

 "cleat") ; why, it may, and has often been asked, does the "cleat" 

 (the direction of the main joints) usually run roughly N.N.W. and 

 S.S.E., this being the general trend, not only in England but in the 

 United States, and probably in many other countries? I have not 

 come across any good reason in explanation of this fact, but reflecting 

 on the point it occurred to me that possibly the following theory 

 might account for it. 



As the Coal-measures were upheaved or elevated at the end of the 

 Coal period, the rocks would cool and consequently contract to some 

 extent, and in contracting would crack, and thus the joints would be 

 formed; but the cause of the joints taking lines roughly parallel 

 with the earth's axis, or closely corresponding with polarity or 

 longitude, I venture to think may have been due to the increased 

 rotary velocity or greater centrifugal force acting upon the coal- 

 seams as elevation proceeded — as they got further and further away 

 from the earth's centre and so became more liable to open, split, 

 and expand : in other words, the tensional strength of the coal gave 

 way along approximate N. and S. lines due to increased velocity of 

 travel as a consequence of elevation and cooling. 



Mathematicians and physical scientists may possibly demonstrate 

 my theory to be contrary to the laws of natural science. If they 

 do, we must then look for some other explanation of the phenomenon 

 of "cleat." At any rate it is hoped that this communication will 

 be accepted or rejected on its merits or demerits, and that it will call 

 up some criticism. 



W. S. Gresley, F.G.S. 



Erie, Pa., U.S.A., lUh Aug., 1892. 



