Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/58

Rh 48 G A L [SEQUENCE OF STRATA. in the production of the very thick masses of coal of small extent found in some coal-fields in Southern Europe. Another theory, that proposed by Dr Mohr, deserves notice, namely, that coal may be of marine origin and derived from the carbonization of sea weeds, such as the great kelp plant of the Pacific Ocean. This has been very ingeniously elaborated by the author, and much apparently .rood evidence adduced in support (see his Geschichte ckr Erde Bonn, 1875). But the positive evidence afforded by roots found in the under clays is sufficient to render such an hypothesis unnecessary in the majority of instances. It must be remembered, however, that, although cellulose or wood fibre is most probably the chief material concerned in the production of coal, this substance is readily con vertible into dextrine by the action of protein or analogous fermentescible matters containing nitrogen, a change that is attended with the loss of structure, the fibre being con verted into a gummy mass. Some forms of cellulose, such as that in the lichens known as Iceland moss, are soluble in water, and are without fibre. The preservation of recogniz able woody tissue therefore in coals can only be regarded as accidental, and any argument founded upon the relative quantity of the recognizable vegetable structures in mi croscopic sections is likely to be unsound, unless the rela tive durability of the different portions of the plants be taken into account. Thus the bark of trees is, as a rule, less perishable than the solid wood, while tissues im pregnated with resinous matters are almost indestructible by atmospheric agency. Instances of this are afforded by the fossil trees found in the coal measures, which are often entirely converted into siliceous masses, the bulk of the wood having decayed and been replaced by silica, while the bark is represented by an external layer of bright coal. Fossil resins, such as amber, are of common occurrence in coals, especially those of secondary or tertiary age. In an investigation of the coking properties of the Saar- briicken coals by Schondorff, it was found that they could be separated into three different materials, which he dis tinguished as glance or bright coal, dull or striped coal, and fibrous coal. The last, which is known in England aa &quot;mother of coal,&quot; resembles a soft, dull, black charcoal, containing abundant traces of vegetable fibre, and yielding a high proportion of non-coherent coke, behaving, in fact, like charcoal. The bright or glance coal is without any apparent structure, cleaving into cubical masses, contains but little mineral matter, and yields a strong coke. The striped coal consists mainly of a dull substance, with fine alternations of bright matter, and is essentially a gas coal yielding only an inferior coke. These differences are sup posed to be due to original differences in the substances from which the coals have been derived. Thus the fibrous coal may result from unaltered cellulose, the glance coal from the insoluble mucilage derived from the maceration of the plants in water, and the dull coal from the soluble parts, such as gum and dextrine, either original or produced by the transformation of cellulose and starch. That some thing analogous to a pulping process has gone on in the pro duction of coal is evident from the intimate intermixture of the mineral matter constituting the ash, which is quite unrecognizable before burning in the majority of instances. F. Muck (Chemische Aphorismen iiber Steinkoklen, Bochum, 1873) has recorded some interesting experiments on the behaviour of the three isomeric carbohydrates, cellulose, starch, and gum arabic, which are all of the same ultimate composition, namely, C (! H 10 5 . When sub jected to the process of coking, cellulose, in the form of Swedish filter paper, gave a residue of 6-74 per cent, of a perfectly non-coherent coke, starch 11-30 per cent, of a bright vesicular coke like that from strongly coking coal, and gum-arabic 2042 per cent, of a hard dull coke re sembling that produced from imperfectly coking gas coals. The volume of gas given off by cellulose and starch is? much larger and of a higher illuminating power than that produced from gum under the same conditions. The conditions favourable to the production of coal seem therefore to have been forest growth in swampy ground about the mouths of rivers, and rapid oscillation of level, the coal produced during subsidence being covered up by the sediment brought down by the river forming beds of sand or clay, which, on re-elevation, formed the soil for fresh growths, the alternation being occasion ally broken by the deposit of purely marine beds. We might therefore expect to find coal wherever strata of estuarine origin are developed in great mass ; and this is actually the case, the Carboniferous, Cretaceous, and Oolitic series being all coal bearing horizons, though in un equal degrees, the first being known as the coal measures proper, while the others are of small economic value in Great Britain, though more productive in workable coals on the continent of Europe. The coal measures which form part of the Palaeozoic or oldest of the three great geological divisions are mainly confined to the countries north of the Equator, Mesozoic coals being more abundant in the southern hemisphere, while Tertiary coals seem to be tolerably uniformly distributed irrespective of latitude. The nature of the coal measures will be best understood g e qu&amp;lt; by considering in detail the areas within which they occur of cai in Britain, together with the rocks with which they are ifer l most intimately associated. The general succession of these &quot; rocks is given in fig. 1 (cols. 1 to 4), which is taken from Q Z tiTUU] .j Burner z |, SS SB 1 v& Sandstone ^ H g Smz g^ Uj Vlagnesian S - &amp;lt; ?H Lfmcston e w & a F fc JC-5 Upper Mcasum .imestmies
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pirortis Lr aitij&quot; lidefmd Rocfr Hi l/jjpcr thin coa/jj T.--- tKin coals] ^ Upper __- kcTtwarthKoch 3f Red Sandstone =3 Middle Middle

Measures Is Series ^ thick coals] Measures thrck coa(s] Middle HE

Loweror Gamster IB Lower or

Measures ra SHAFTON COAL 5.7 - -- Cool

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Grit -- Midstot ie __ Measures

Sw|wroN COAL 5.6 STEAM&quot;COAL - y Series Grit

vVoojJHE^RA S Roslyn Sandstone ii Sandstone & sha/cs

Scries

Millstone Grit

WINTER COAL 4.0 KENTS THIN I.ID ~_- or j_; &quot; -S3&amp;lt; KENT S THICKtt HI: Moor Rock

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BARNSLEYCOAL.9.4 H Carlonffcrous z:

Upper Limestone

Jpper &amp;lt;J ! SWAILQWW0003.2 COAL Limestone Scries Tlfe Liniesrone Shale Series Shale | FLOCKT(VJCOAL3.&amp;lt; iv --&quot; -- Ill i- r.._ &quot; RWKGATE 5.4 ==s [Edqe Coal l[l] THORNCLIE SarUt]



Curbont/- FOUR FOOT 11 3.i ^ with shales aiidLimestoncs ^ J jm Carioni/5 LJL ^r: SILKSTONE 5,0 JTT-^T 111

Series II Limestone Limcsfont WHIN MOOR &quot;3.0 u j UPPER PSNISTONE WS CxlcifefoVS . : i Sandalonfs i Scries ill! C - COftL GPEENSIDE OOAL , f? Sandstone ^^ &quot; 1 i i ~^ ^ Series Sane? stones . shales il M g Creenmeor IEEI with

with E Sandstone Oil Shales - Limestones / ft 1 ^_-._ UPPER BAND in upper &quot;. nil L CCAL part 1 CT on ( OUtC *&quot; HAPDBEDCO/II.S -. . ^ 1 -- Cv--. - - GANKTER CPAL B Old Red Sandstone /i o Lower St seen in rat, tin 1C g Shale &quot;ej OM Red Sancfsfonc &quot; - SOFT BED COW. 1.6 CLAY COAL Rough Rock 1 [Dei/onian] area 8 ~ .&quot; [Oci/071KUl] r i Millstone Grit FIG. 1. Succession of Carboniferous Strata. the index of strata issued by the Geological Survey. The
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