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In Nassau and Bavaria woody structure is very common, | and it is from this circumstance that the term lignite is derived. The best varieties are black and pitchy in lustre, or even bright and scarcely to be distinguished from true coals. These kinds are most common in Eastern Europe. Lignites, as a rule, are generally found in strata of a newer geological age, but there are many instances of perfect coals being found in such strata.

By the term "ash" is understood the mineral matter remaining unconsumed after the complete combustion of the carbonaceous portion of a coal. This represents part of the mineral matter present in the plants from which the coal was originally formed, with such further addition by infiltration and mechanical admixture as may have arisen during consolidation and subsequent changes. The composition of the ashes of different coals is subject to considerable variation, as will be seen by the following list of analyses :TABLE II.-Composition of the Ashes of Coals.

The composition of the ash of true coal approximates to that of a fire-clay, allowance being made for lime, which may be present either as carbonate or sulphate, and for sulphuric acid. The latter is derived mainly from iron lphur in pyrites, which yields sulphate by combustion. An indication of the character of the ash of a coal is afforded by its colour, white ash coals being generally freer from sulphur than those containing iron pyrites, which yield a red ash. There are, however, several striking exceptions, as for instance in the anthracite from Peru, given in Table I., which contains more than 10 per cent. of sulphur, and yields but a very small percentage of a white ash. In this coal, as well as in the lignite of Tasmania, known as white coal or Tasmanite, the sulphur occurs in organic combination, but is so firmly held that it can only be very partially expelled, even by exposure to a very high and continued heating out of contact with the air. An anthracite occurring in connection with the old volcanic rocks of Arthur's Seat, Edinburgh, which contains a large amount of sulphur in proportion to the ash, has been found to behave in a similar manner. Under ordinary conditions, from to of the whole amount of sulphur in a coal is volatilized during combustion, the remaining to being found in the ash.

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The amount of water present in freshly raised coals varies very considerably. It is generally largest in lignites, which may sometimes contain 30 per cent. or even more, while in the coals of the coal measures it does not usually exceed from 5 to 10 per cent. The loss of weight by exposure to the atmosphere from drying may be from to of the total amount of water contained.

Coal is undoubtedly the result of the transformation of vegetable matter, mainly woody fibre, by the partial elimination of oxygen and hydrogen giving rise to a substance richer in carbon than the original wood,-the mineral matter being modified simultaneously by the almost entire removal of the alkalies and lime, and the addition of materials analagous in composition to clay, as will be seen by comparing the analyses in Table II. The

Mohr has computed that the transformation of wood into coal is attended with a loss of about 75 per cent. in weight; and, having regard to the difference in density of the two substances, the volume of the coal can only be fromto of the woody fibre from which it is derived.

The nature of the change is essentially a slow oxidation under water or any covering sufficient to protect the dead wood from the direct action of atmospheric air, as in the latter case the vegetable mould or humus would be produced. The products of such decomposition vary with the length of time and the nature of the plants acted on, and in the case of anthracite the change is so great that no portion of the original plant structure can be recognized, at the same time the density and conductivity for heat and electricity are increased. This, however, is a case of metamorphosis analogous to the transformation of sedimentary into crystalline rocks, the extreme term of such metamorphosis being the production of graphite or plumbago. Daubrée has shown that wood may be converted into anthracite by exposure to the action of superheated water at a temperature of 400° C.

The plants concerned in the production of coal vary very Coal-pro considerably in different geological periods. In the coal ducing allied forms are most abundant. It is stated by some measures proper, acrogens, ferns, equisetums, and similar plants. observers that entire beds of coal are sometimes made up of the spores of ferns. This, however, appears to depend upon the inspection of microscopic sections, and may not be capable of rigorous quantitative demonstration. In the coals of newer date exogenous wood and leaves are more common than in those of the coal measures; the former also contain resins, sometimes in considerable quantity.

The number of species of land plants in the British sedimentary formations, which may be taken as a measure of the comparative prevalence of coal in the different series, is as follows:

The most generally received opinion is that much if not all coal results from the transformation of plants upon the site of their growth. The principal evidence in favour of such a supposition is afforded by the common occurrence of a bed of clay, the so-called "under-clay," containing the roots of plants, representing the old soil, immediately below every coal seam-a fact that was first pointed out by the late Sir W. E. Logan in South Wales. In Yorkshire the same thing is observed in the siliceous rock called ganister occurring in similar positions, showing that the coal plants grew there upon sandy soils.

The action of water in bringing down drift wood may have also contributed some material, but much less than the local growth. This may probably have been concerned

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 good evidence adduced in support (see his Geschichte der 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 convertible 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 converted 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 recognizable 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 microscopic sections is likely to be unsound, unless the relative 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 impregnated 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 Saarbrücken coals by Schondorff, it was found that they could be separated into three different materials, which he distinguished as glance or bright coal, dull or striped coal, and fibrous coal, The last, which is known in England as "mother of coal," 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 supposed 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 something analogous to a pulping process has gone on in the production 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 über Steinkohlen, 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, CH10O5. When subjected 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 20.42 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 occasionally 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 unequal 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 Palæozoic 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 Sequen by considering in detail the areas within which they occur of carben in Britain, together with the rocks with which they are iferous most intimately associated. The general succession of these rocks is given in fig. 1 (cols. 1 to 4), which is taken from

strata

the index of strata issued by the Geological Survey. The