62 SECTIONAL ADDRESSES. 



because it may mean that a thick seam may in the divided condition 

 become incapable of being worked at a profit. 



The great coalfield that I have so often cited furnishes examples of 

 every known type, and interesting as they are to the geologist, they 

 are an abomination to the colliery-owner or manager, and often a 

 source of severe disappointment and loss. The most notable split seam 

 in Britain is not, however, in Yorkshire but in the famous Staffordshire 

 Thick Coal. Jukes showed that this magnificent seam, 40 feet thick 

 at its maximum, is split up into a number of minor seams by wedges 

 of sedimentary strata which aggregate, in a distance of 4i- miles, a 

 thickness of 500 feet. Whether these intercalations again thin out, or 

 not, is unknown to me; but whether so, or not, the explanation offered 

 by that sagacious student of coal. Bowman of Manchester, might find 

 here a typical application. Bowman supposed that a local sag occurred 

 in the floor of the coal swamp, resulting in the drowning of the vegeta- 

 tion (in his illustration bearing a suspicious resemblance to a coconut 

 palm) and interrupting the formation of peat until the hollow was silted 

 up and a new swamp flora re-established. This explanation remained 

 for many years unchallenged, but in 1875, in the great memoir on Tlie 

 Yorkshire CoaJfiehl, Green advanced a new reason for the splitting of 

 seams, which is a very common phenomenon here, scarcely any, if 

 any, seam being exempt. 



Green pointed out that as the Silkstone seam is traced northward 

 from the locality near Barnsley with which its name is associated, it 

 begins to exhibit partings of ' dirt,' which thicken to a belt of country 

 where no collieries afforded information as to the behaviour of the seam. 

 On tlie far side of this gap a seam is found on the same horizon, 

 but if it represents the Silkstone seam it is very much attenuated and 

 divided. He attributed these features to the development, contem- 

 poraneously with the accumulation of the measures, of a ridge of land, 

 whence mud was washed into the coal-swamp on either hand. Later 

 in the same volume exactly the same problem is presented by the 

 Barnsley Bed, which deteriorates in just the same manner in an 

 almost identical geographical position. This was hailed by Green as 

 a further example of the same process. 



So long as the problem was of merely academic interest I was content 

 with a silent demurrer, but having to consider the probable resources of 

 the debatable ground for the purpose of colliery development I sought 

 criteria with which to decide whether Green's growing anticline or 

 Bowman's developing syncline was the correct explanation. This was 

 the more necessary as I found that the tendency to split affected seams 

 still higher than those named. Now, it will be obvious upon reflection 

 that an anticline undergoing intermittent elevation and denudation should 

 cause a convergence of the strata representing the stationary phases as 

 they approach the axis, while a deepening trough should produce a 

 corresponding divergence of the strata — principles well illustrated by the 

 Market Weighton and Cleveland axes respectively. A careful plotting 

 of intervals showed that, selecting the two seams that were most gener- 

 ally worked, isopachytes of the strata separating them could be drawn, 

 and Bowinan's sag demonstrated. 



