256 



KNOWLEDGE 



[November 1, 1890. 



Some interesting calculations as to the time required 

 for the formation of the coal-measures are given. On 

 the hypothesis of t/roiiUi in situ, and a gradual sinking of 

 the surface, 800,000 years would be required ; for the 

 hypothesis of deltas, 16,000 years; ht diandc Cinic/ir, on 

 the former view, would require 318, .500 years, and on the 

 latter, 2,500. 



On arriving at the question of the origin of coal, M. 

 Fayol asserts that all the varieties in the coal-field are 

 formed directly by vegetable dehris similar to that usually 

 carried by streams. Vegetable fragments found car- 

 linnized in shales and sandstones are usually ascribed to 

 ilnft, and since there is a gradual passage from such beds 

 to pure coal, there is no reason to doubt that these latter 

 may be formed in the same way. 



A bed of conglomerate 26 ft. thick, intercalated with 

 the chief coal-seam, presents great difficulties on the 

 theory of original horizontality and growth in situ, but is 

 easily explained on the hypothesis of deltas. An un- 

 usually severe flood carries coarse particles beyond the 

 ordinary resting-jjlace of such, and deposits them on a 

 bed of vegetable flchris ; things return to their usual 

 course, and nothing but vegetation is laid down on the 

 gravel. Floods and changes of watercourses cause the 

 coarser sediments to invade the finer ; the latter are then 

 found intercalated with the former. The numerous local 

 disturbances which occur are easily explained as the result 

 of the pressing out of the soft incoherent beds of mud by 

 the weight of the sediment above. 



The pebbles of granite found in the coal itself are 

 thought to have been carried by the trees which form 

 it. When the rivers cut through some already bedded 

 vegetable matter, this, being denser by reason of the change 

 already undergone, is deirosited with the sand, and 

 " black sandstones " are the result. 



The cleavage of the coal, M. Fayol explains as a pheno- 

 menon of contraction. Upright trees occurring in coal 

 liave usually been taken as indications of growth in situ ; 

 and since such occur in the coal-basin of Commentry, it 

 becomes necessary to show that they do not necessarily 

 imply this. 



M. Fayol shows that i(/iri(ilit trunks form only a small 

 percentage of the large number occurring in his special 

 ar(;a ; as the beds become coarser and less carbonaceous, 

 the proportion of upright trunks increases ; the 

 rocks which contain the fewest trees have the largest 

 proportion upright. And some of these perpendicular 

 trunks are upside down, with their roots in the air ! 

 Many are without roots, and others have their roots 

 damaged and bruised. Moreover, the roots of these up- 

 right trees have often greatly afl'ected the stratification of 

 the beds in which they lie ; roots growing in the soil 

 would not do so. 



To ascertain the efl'ect of heat and pressure on sediments 

 containing vegetation, some experiments have been tried. 

 Various animal and vegetable remains have been placed 

 with water in tubes, boilers, &c., for various periods of 

 time, and sometimes enveloped in sand, mud, &c. The 

 following results have been obtained : At ITr to 20° C, 

 under a pressure of 65 yards of water, an infiammable 

 gas has been produced. Organic matter protected from 

 the air decomposed very slowly ; the rate of decomposi- 

 tion grows slower as time passes. Woody fibres, fibrous 

 bark, and the epidermis of leaves and grains resist best ; 

 cellular portions, young branches, interior of grains, &c., 

 decompose first. Weight is lost, and the residue becomes 

 more carbonaceous. The alteration is about the same, 

 whether the vegetable matter is in contact with water, or 

 enveloped in earthy sediments. 



One of the must interesting sections of the book is that 

 devoted to the pluiiuiiiri f sedimentation. In his nume- 

 rous and beautiful e\|iernnents, M. Fayol has produced in 

 miniature every pecuharity of the coal-field of Com- 

 mentry. As a general conclusion, both from natiu'al and 

 artificial deltas, M. Fayol divides each into two parts, wliich 

 he terms Alluvial and Neptunian respectively. The latter 

 consists of beds Lachned at angles of fi-om 0' to 15", and 

 the I'ormer are spread over them horizontally, or nearly so. 



In tranquil water the coarser materials are de2)osited in 

 steep slopes, as if they had been tipped from a waggon ; 

 the finer sediments in agitated water are spread over 

 greater distances and may be nearly horizontal. The dip 

 of the beds increases with the coarseness of the mate- 

 rials, the feebleness of the current, the smallness and 

 the tranquillity of the basin, and rici' irrsd. 



Sometimes a third set of beds, nearly horizontal, and 

 b('l(nr the Neptunian part, may be observed. 



Highly inclined beds of coarse grain, then, are likely to 

 have been formed in lakes fed by Alpine torrents ; hori- 

 zontal beds of fine grain point to fiuvio-marine conditions. 

 Hence the inference that the irregular, inclined, and 

 coarse beds of the coal-fields of Commentry have been 

 formed in an Alpine lake ; while the more extended, finer, 

 and more horizontal strata of the northern French coal- 

 field are ascribed to fiuvio-marine conditions. 



M. Fayol claims that there is not a single peculiarity of 

 the coal formations which is not to be found in the deltas 

 of the present day, and imitated in artificial depo.sits. 



These experiments and observations on .sedimentation 

 have a value far beyond their immediate application to the 

 coal-fields of Central France. They will be found of 

 great value in explaining the general phenomena of strati- 

 fication. 



If M. Fayol's theory is correct, it follows that the 

 inclination of the strata in the coal-field of Commentry is 

 nat due to subsequent earth movements, but is the 

 original position of the Neptunian part of the delta. 

 Consequently, he has to do battle with the geological 

 axiom ot the original horizontality of aqueous deposits. 



M. Fayol goes into the history of this hypothesis, and 

 points out that it was first formulated by Stenon, a native 

 of Denmark, in 1669. 



The fact that fiat pebbles in inclined strata lie parallel 

 to the bedding, is considered one of the strongest proofs. 

 But, as we have seen, this is the natural position taken in 

 beds originally inclined. 



The horizontality observed in alluvial beds at the 

 mouths of rivers is another supposed proof. These, how- 

 ever, are only the horizontitl corcrinii of the inclined Nep- 

 tunian beds. 



The regular thickness of marine beds, and the feeble 

 inclination of the sea bottom, is thought to show that all 

 beds must be nearly horizontal when first laid down. 

 But a gentle inclination of the bottom does /iuf prevent 

 the formation of xtirjdy inclined beds near the mouth of 

 a river. Thus the bottom of the Mediterranean has a 

 feeble slope, and yet, at the mouth of the Var, deposits, 

 inclined at angles of from 25° to 30', are formed. A 

 similar inference is to be drawn from the experiments. 



Nor do beds of vegetable matter at various heights in 

 deltas iicccssdiili/ imply horizontality ; they are found at 

 all inclinations between O'and 10", andean, as experiment 

 shows, be formed by vegetation carried by streams. 



The convenient hypothesis of movements of the earth's 

 crust has, perhaps, been too freely invoked for the expla- 

 nation of geological phenomena ; it is well to have it thus 

 brought home to us, that in some cases we may dispense 

 i with it. 



