AND OTHER VARIETIES OF COAL. 117 



that the striae of wood are produced by fibres and vessels, but 

 the striae of coal are due to its formation of laminsp. 



We now return to our examination of the appearances of 

 thin sections of Torbanehill coal : — 



Many of the larger yellow spots of horizontal sections 

 (Plate VII., fig. 1) are lobed, as if compounded of three or more 

 smaller spots joined together. In the greater number of these, 

 more or less distinct double lines^ which radiate Irom ttie 

 centre of the spots, divide them into a number of polygonal 

 spaces, more highly coloured than the spaces between the 

 double lines, and often presenting at some part a still darker 

 spot in each. This delicate division of the yellow matter 

 into polygonal masses is most obvious when preparations are 

 preserved in Canada balsam, and almost entirely disappears 

 when sections are preserved and examined in fluid. A similar 

 radiate striation exists in the corresponding reddish-yellow 

 bodies of the Wemyss coal (Plate VII., figs. 5 and 6). But it 

 happens very commonly that the dark-brown matter of the 

 boundary of these spaces stretches into them for some distance 

 in lines at times reaching to their centres, where there are 

 other dark-brown lines arranged in a radiate (often a tri- 

 radiate) manner. These lines are quite permanent in whatever 

 medium the sections may be examined. 



The action of heat offers great assistance in the determina- 

 tion of the nature of the yellow spots. It dissipates the whole 

 of the yellow colour, and leaves angular and polygonal open- 

 ings in the thin section, bounded by the dark-brown matter, 

 which it has slightly blackened (Plate IX., fig. 2). It can now 

 be determined, even upon a single section, that the boundaries 

 of the yellow matter form definite walls for all the smaller 

 spaces, for where the section has passed directly through the 

 wall, this is shown by a broad, sharp, black line; but where 

 it has cut the yellow mass near its upper or lower wall, there 

 is a shading from the edge of the spot towards its centre, as 

 would be the case if the boundary were formed of vegetable 

 membrane. But further, in such yellow spots as present no 

 very clear indication of the existence of membranous septa in 

 their interior, heat often renders the existence of such septa 

 as obvious as in other parts. A yellow spot is shown in 

 fig. 2, from half of which the yellow matter has been entirely 

 driven off, whilst it remains in the other half. In the half 

 least acted on by the heat, this agent has blackened septa 

 which before were scarcely visible ; and, in the other half, such 

 septa are seen to form the boundaries of spaces of a regularly 

 angular and polygonal shape, and of the same size as those 

 which constitute the smaller yellow patches of fig. 3, Plate VII, 



