Sept. 26, 1884.] 



KNOV/LEDGE ♦ 



255 



NOTES ON COAL. 



By Richard A. Proctor. 



(^Continued from p. 217.) 



■XTTEsee, then, that coal-seams are the remains of ancient 

 V \ vegetable layers, formed underneath the trees of 

 the ancient forest. But it is not to be supposed that every 

 forest in those old times spread its shade over a ma^s of 

 decaying vegetable matter, until the time should come 

 when the mass should be covered over with shale or sand- 

 stone. In order that coal-seams should be formed, it was 

 necessary that the forest region sliould te so abundantly 

 watered as to form a forest swamp, like the cypress-swamps 

 ai the Mississippi. Yet again, it was necessary that 

 during the fresh-water inundations which helped to accu- 

 mulate the vegetable matter lound the roots of the ancient 

 forest trees, no mud should be carried into the swamps. 

 As Lyell says, " One generation after another of tall trees 

 grew with their roots in mud, and their leaves and pros- 

 trate trunks formed layers of vegetable matter, which was 

 afterwards covered with mud, since turned to shale. Yet 

 the coal itself, or altered vegetable matter, remained all 

 the while unsoiled by earthy particles." This is a fact 

 which seems at a first view altogether perplexing ; but, as 

 uearly always happens with the more perplexing features 

 of any natural enigma, geologists have been led by this 

 difficulty to the interpretation of the enigma. It is to 

 this very fact that we owe the most trustworthy infor- 

 mation yet obtained respecting the process by which 

 coal-beds were originally formed. The solut ~n is 

 due to the same eminent geologist from whom I have 

 already quoted the statement of the difficulty. " The 

 enigma," be says, '^ however perplexing at first sight, may, 

 { think, be solved by attending to what is now taking place 

 in deltas. The dense growth of reeds and herbage which 

 encompasses the margins of forest-covered swamps in the 

 valley and delta of the Mississippi, is such that the fluvia- 

 tile waters, in passing through them, are filtered and made 

 to clear themselves entirely before they reach the areas in 

 which vegetal le matter may accumulate for centuries, 

 forming coal if tie climate be favourable. There is no 

 possibility of the least intermixture of earthy matter in 

 such cases. Thus, in the large submerged tract called the 

 " Sunk Country," near New Madrid, forming part of the 

 western side of the valley of the Mississippi, erect trees 

 have been standing over since the year 1811-1812, 

 killed by the great earthquake of that date ; lacustrine 

 and swamp plants have been growing there in the shal- 

 lows, and several rivers have annually inundated the 

 whole space, and yet have been unable to carry any 

 sediment within the outer boundaries of the morass, so 

 dense is the marginal belt of reeds and brushwood. It may 

 be affirmed that generally in the " cypress-swamps " of the 

 Mississippi no sediment mingles with the vegetable matter 

 accumulated thein? from the decay of trees and semi-aquatic 

 plants. As a singular proof of this fact, I may mention 

 that whenever any part of a swamp in Louisiana is dried 

 up, during an unusually hot season, and the wood is set on 

 fire, pits are burned into the ground many feet deep, or as 

 f-ar down as the fire can descend without meeting with 

 water ; and it is then found that scarely any residuum or 

 earthy matter is left At the bottom of all these " cypress- 

 swamps " a bed of clay is found, with roots of the tall 

 cypress, just as the under-clays of the coal are filled with 

 stigmaria — the roots of the ancient forest-trees called 

 iigillaria* 



* It is not quite certain to what type of vegetation these trees 

 belonged, 'i bey were formerly supposed to be tree-ferns, hut some 



It will be seen that the circumstances here considered 

 dispose of the theory — once a favourite one with many 

 geologists — that the coal-seams were formed of vegetable 

 matter (the rubbish of decayed forests) which had been 

 carried by rivers into estuaries, and there formed into vast 

 natural rafts. It was supposed that such rafts, sinking to 

 the bottom, became afier awhile covered with a layer of 

 sand or mud. The uprightness of the tree-stumps, how- 

 ever, as compared with the position of the coal-beds — that 

 is to say, their position square to these beds — should of 

 itself have disposed of the theory referred to. 



Yet, on the other hand, there is great difficulty in 

 understanding under what circumstances the alternate 

 rising and sinking of the level of these delta-swamps, 

 or morasses, took place during the enormously long 

 period of time which must have been occupied in the 

 formation of the carboniferous groups with a thickness 

 amounting in some places to nearly four miles. We see, 

 for instance, that in the case of the Nova Scotia coal- 

 fields there must have been eighty-one distinct sub- 

 mergencies. Now there is nothing remarkable in the mere 

 circumstance that the same part of the earth should have 

 been above and beneath the sea-level through many suc- 

 cessive alternations. Geology has long taught that in 

 nearly every part of the earth this must have happened ; 

 but that throughout so many as eighty-one such changes 

 those conditions should have been repeated which are 

 necessary for the formation of coal-beds is ir.deed a most 

 remarkable circumstance. We have, on the one hand, 

 the ind ^,itions of a surprising degree of subterranean 

 activity , for whether the land sank or the sea rose, there 

 must have been a great oscillation of the earth's crust. But, 

 on the other hand, we see that the great swamps must 

 have retained their horizontal position unaltered for long 

 periods of time. The growth of a forest is not the work 

 of a few years, nor could the accumulations of vegetable 

 matter have been formed quickly. As Lytll says, we have 

 "evidence of the former existence at more than eighty 

 different levels " — overlying levels, be it noticed — " of 

 forests of trees, some of them of vast extent, and which 

 lasted for ages, t:i\ing rise to a great accumulation of 

 vegetable matter" Under what condition must the 

 earth's crust have been when such proce^ses were possible? 

 To this question, as yet, geology has given no satisfactory 

 answer. There are considerations, however, which seem 

 at least suggestive of a solution of some of the diffi- 

 culties here presented. 



{To be continued.) 



Mr. Mackay, of the Eoyal School of Mines, is announced to give 

 introductory (free) popular scientific lectures at the Highbury 

 Institute, on Sept. 25 and 30. Mr. G. Hawker, too, commenced a 

 series on the " Harmony of Form, as Applied to the Decorative 

 Arts," at the City of London College, on Tuesday evenings. Par- 

 ticulars in the College prospectus. 



are found to have had long straight leaves, unlike those of ferns. 

 The reader will probably remember that, after describing the sigil- 

 laria, the author of " Vestiges of Creation" describes the stigmaria 

 as a distinct plant. " Among the most remarkable of the leading 

 plants of the coal era, without representatives on the present sur- 

 face, are the sigillaria, of which large stems are very abundant, 

 showing that the interior has been soft, and the exterior fluted, 

 with separate leaves inserted in vertical rows along the flutings ; 

 and the stigmaria, a plant apparently calculated to flourish in 

 marshes or pools, having a shorr, thick, fleshy stem, with a dome- 

 shaped top, from which spring branches of from twenty to thirty 

 feet long." These branches were, in realiry, the roots of the 

 sigillaria. The mistake is a very natural one, since the coal-seam 

 actually separates the trunk of the tree from its roots. Some, 

 however, have since been found attached to the base of the tree- 

 stumps. 



