54 SECTIONAL ADDRESSES. 



more abundant in some seams than in others, is dull in aspect with a 

 brownish tinge, sometimes even being of a deep coffee colour. It is of 

 rougher texture, breaking with a rather hackly fracture. The third 

 material, commonly known as ' mother of coal ' — a name innocent of 

 misleading implications, for which the French name ' fusain ' is often 

 substituted — is disposed in thin layers between the other constituents. 

 The fusain layers are of weak texture, so that the coal when struck 

 almost invariably splits along one of these. Upon the cleft surface a 

 soft charcoal-like substance is seen, made up apparently of broken 

 fragments. It is this which makes coal dirty to handle.^ 



What is the meaning of these three types of substance constituting 

 the seam ? Even with the unaided eye we may gain a clear reply to 

 our question, but the microscope gives a better answer, and discloses 

 many interesting details. The highly lustrous coal was long ago recog- 

 nised by Dawson as being produced from the bark of trees, and it is 

 common experience that isolated shells of bark with the characteristic 

 external leaf-scars and internal marks of leaf-traces are usually of 

 bright coal. This, however, is probably not quite the whole of the 

 explanation of the bright coal, for I have in my possession specimens 

 in which a larger fraction of the original radius of a trunk than can be 

 ascribed to bark in the most elastic sense of the term is represented by 

 coal of extraordinary brilliance. Again, where coal has undergone 

 great disturbance prior to its ' mineralisation ' it is usual to find a 

 large development of bright coal. However, it is a great aid to the 

 interpretation of a seam to know that the long bright streaks do usually 

 represent in some shape the trunks or branches of trees. The dull 

 coal may be a felt of the finer elements of plants, or, in Lomax's phrase, 

 mixed humic debris, but few, if any, of the seams in the Yorkshire 

 Coalfield fail to include layers of which the main constituents are the 

 spores of lycopods — both megaspores and microspores. With a glass 

 of even low magnifying power we may recognise in hand specimens 

 of coal the small discs which represent the flattened spores, each with 

 a triradiate mark indicating its contact with the other three members of 

 the tetrad. In thin section for the microscope they appear as yellow 

 discs, or sacs, sometimes in horizontal sections showing the three- 

 rayed ridge. The methods by which these plant-spores have been 

 accumulated may have differed. The spores may have been wafted 

 from distant jungles of SigilJaria or kindred trees, though their large 

 size is rather opposed to the view of wind carriage from a considerable 

 distance, and it seems the more probable supposition that they accumu- 

 lated on the ground or on the carpet of vegetation in the water beneath 



1 Dr. St opes has proposed [Pror. U.S., Ser. B., xc, p. 470) a classification of 

 constituents of coal-seams into four types : 1, Fusain, the familiar ' mother of 

 coal ' ; 2, Durain, which includes the spore coals (generally these belong to the 

 'hards' in a seam); 3, Clarain, which apparently includes the humic coal of 

 Lomax ; and 4, Vitrain, a very brilliant coal forming thin bands and showing a 

 complete a))sence of structure in typical specimens. Mr. 'Sinnatt (Tran.^. Iii-if. 

 Aim. J'j'i'j.. vol. Ixiii., p. 307) adopts thc^e names and proposes a system of con- 

 vention.il shading by means of which they may be distinguished in drawings. 

 He also attempts an analysis of tlie proportions of each type in certain coal- 

 seams in Lancashire. 



1 



