December 16, 1922] 



NA TURE 



811 



The Physiography of the Coal-Swamps. 1 



By Prof. Percy Fry Kendall, M.Sc, F.G.S. 



'"PHE subject of Coal Measures geology has been 

 -L discussed piecemeal in innumerable papers and 

 memoirs, so that an inquirer may well be appalled at 

 the mass of facts and of often conflicting deductions 

 with which he is confronted. Indeed, it is surprising 

 to discover how fundamental are some differences of 

 opinion which exist. 



Among the questions in the answer to which doctors 

 have differed there is, I imagine, none more funda- 

 mental than this : 



Were coal seams simple aggregations of plant remains 

 swept together by the action of water — a process of 

 accumulation which the learned call allochthony ; 

 more simply by drift ; or were they formed, like 

 peat, by the growth of vegetable material in its place 

 — the process of autochthony ? 



I do not intend to labour the answer to this question. 

 Categorical arguments in favour of the growth in place 

 origin of the coal-forming vegetation are on record, 

 and they have never 'been as categorically answered. 

 Many arguments in favour of the drift theory seem 

 to me clearly to have arisen from confusion between 

 cannel and true coal. This distinction is again funda- 

 mental. True coal-seams are characterised by : — 



(1) Wide extent. 



(2) Uniformity of thickness and character over 



extensive areas. 



(3) Freedom from intermingled detrital mineral 



matter. 



(4) Constant presence of a seat-earth or rootlet bed. 



(5) Entire absence of remains of aquatic animals 



within the seam. 

 Substitute affirmatives for negatives, and negatives 

 for affirmatives, and the characteristics of cannel are 

 as truly set forth. 



The Aberrations of Coal-seams. 



Having got our coal-swamp clothed with vegetation, 

 and the coal-forming materials accumulating, let us 

 next consider the various interruptions of continuity 

 and the aberrations to which it is liable. These 

 interferences may be either contemporaneous with 

 the accumulation of the materials, or, as one may say, 

 posthumous. 



Prominent in the category of contemporary interfer- 

 ences must be put the phenomena of split-seams. 

 A split-seam is the intercalation into the midst of the 

 coal of a wedge of sandstone, shale, or the like, in such 

 wise that the seam becomes subdivided by intervening 

 strata into two or more seams. The most notable 

 split-seam in Britain is 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 4! miles, a thickness of 

 500 feet. 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 vegetation and inter- 



NO. 2772, VOL. IIO] 



rupting the formation of peat until the hollow was 

 silted up and a new swamp flora re-established. 



I now turn to a form of split-seam of extraordinary 

 interest, which has received comparatively little 

 attention from geologists though mining engineers 

 must surely have a special comminatory formula to 

 express their sentiments thereon. The first example 

 that came under my notice was encountered in the 

 eastern workings of the Middleton Main Seam, at 

 Whitwood Colliery, near Wakefield. Thin intercala- 

 tions of shale and other sedimentary materials, appear- 

 ing at different horizons in the seam, were found to 

 thicken gradually to the east concurrently with the 

 gradual dwindling of the lower part of the seam. An 

 exploration was then carried out. The bottom coal 

 was followed, but it was found that though the under- 

 clay continued the coal disappeared, and was wholly 

 lost for a short distance before it reappeared. The 

 top coal rose over a steadily thickening shale parting, 

 and disappeared into the roof of the workings, but 

 boreholes proved that it was present above a parting 

 which was, at the maximum, 29 feet thick. At the 

 farther end of the heading the top coal came down 

 and the integrity of the seam was restored. Two 

 other transverse explorations have proved the same 

 general arrangement on the same scale of magnitude 

 and one or both margins have been traced for a long 

 distance, enabling the interruption to be mapped 

 continuously for about 8 or 9 miles and intermittently 

 much further. 



My first impression was that this was just a simple 

 case of Bowman's " sag," until I observed that in 

 every traverse the upper element of the seam was arched 

 while the floor was flat. 



Several analogous cases came under my notice before 

 an explanation of this anomalous arching was reached. 

 The explanation was found to lie essentially in the 

 differential shrinkage undergone by peat-stuff in the 

 process of forming coal, and, on the other hand, by 

 any sand or mud which may have been deposited so 

 as to replace a part of the peat. 



Let us imagine a stream being diverted at flood 

 time across a bed of peat and scooping out for itself 

 a hollow channel which subsequently becomes filled 

 with sediments, and afterwards the formation of peat 

 continues, the peat plants creep out, and presently 

 envelop the whole mass of sediments. When the beds 

 consolidate there will obviously be very different con- 

 traction between the sands, muds, and the coal-Stuff. 

 The sands will scarcely contract at all, the muds will 

 contract a good deal, the coal-stuff will contract very 

 greatly. 



Let us now return to the consideration of the plano- 

 convex lens of "dirt"' occupying a position between 

 the upper and lower elements of the split-seam at 

 Whitwood. On the sag explanation it should be 

 convex downward, yet in this as in all other cases I 

 have investigated, it is convex upward. The explana- 

 tion is simple. Let us make our mental picture of 

 the infilled channel in the peat a little more specific 

 in detail. Let us suppose that the peat was 40 feet 

 in thickness* when the river commenced to cut its 



