10 COALFIELDS AND COLLIERIES OF AUSTRALIA. 



are not readily separated, but, say, an inch of coal clings to 

 each side of them. After exposure to the weather, however, 

 the parting may come clean away. Thin bands are not easily 

 separated, so go to increase the ash of coal. The fireclays 

 frequently found below coal seams are believed to be the old 

 soils on which the vegetation grew that is now preserved as 

 coal. The original soil has been deprived of its soda, potash, 

 lime, magnesia, and iron, by leaching, and as these constitute 

 the fluxes, the residue forms a fireclay. 



Coal may vary considerably in its nature and value, accord- 

 ing to its position, as pressure has a great influence upon it. 

 With regard to outcrops, if the surface is badly cut up into 

 deep gullies, the more dirty coal is likely to be found under the 

 long and narrow points, since it withstands disintegration 

 better than good coal, which is more likely to be found in the 

 compact ground. The pressure of superincumbent strata is 

 likely to thicken a plastic bed at its outcrop, as there is little 

 resistance to balance it. This increase of thickness along a 

 line of outcrop in a gully tends to give the strata a local up- 

 ward tendency. Should the roof of the coal be sandstone, while 

 underfoot there is a considerable thickness of slate and clay, 

 then, as the latter are more plastic, and are swelled by the 

 action of the atmosphere, causing great pressure from below, 

 the coal is likely to be thinner at the outcrop than further in. 

 If slate forms both the roof and floor, the coal is not likely to 

 be thus affected. A soft, sooty material at the surface may 

 pass into a coherent, friable and pure material. Hard streaks in 

 coal are more pronounced at the surface, and may give it a dry, 

 dead appearance. Further under cover, such a coal may be- 

 come of fair quality, owing to the increase of volatile and 

 oily substances, and a proportional decrease in ash. A 

 black, friable, slaty material usually passes into an inferior 

 coal. As a general rule, individual coal seams become more 

 tender as depth is attained, and deep lying seams of bituminous 

 coal are more likely to be gassy than near the surface. 



A peculiar spheroidal form of coal known as "coal apple" 

 is of frequent occurrence in the Borehole seam, especially in 

 the Bullock Island, Stockton, and Australian Agricultural 

 Company's collieries, at Newcastle, New South Wales. Such 

 forms have been noted in other parts of the world, and de- 

 scribed*, but so 'far their origin appears to be a mystery. 

 These coal apples occur both in anthracite and bituminous coal 

 seams. The size and shape of the apples vary; the former de- 

 pending, to a great extent, on how the specimen has been 

 trimmed down, the stucture becoming more perfect towards 



*W. S. Gresley, "Note on Anthracite 'Coal Apples' from 

 Pennsylvania" Trans. Am. Inst. Min. Eng., XXI., 824. 



