In cross section the rolls of medium size appear as more 

 or less symmetrical upheavals of the floor shale. The 

 forms and dimensions of some of these are shown in Figs. 

 1, 2, 4, 5, and 6. Pig. 3 shows a double crowned roll. The 

 proximity of such rolls is heralded usually by a slight 

 thickening of the coal, then by a gradual thinning. At the 

 same time the joints or "backs " begin to incline slightly, 

 and this becomes more and more marked as the working 

 progresses up the side of the roll. In many instances, as 

 was pointed out to me by Mr. Thomas Bissell (Mount Keira), 

 a band of pyrites puts in along the floor of the seam. At 

 the crest of the roll the thickness of the coal is reduced 

 below the average thickness by an amount equal to the 

 height of the roll; with rolls of average dimensions, how- 

 ever, the roof is affected not at all or only vcrij slight I ij. 

 At the highest point of the roll the "backs" are again 

 vertical, but they become inclined as we pass beyond this 

 point, so that their arrangement may be described as radial 

 or fan-shaped. That the coal has sunVred considerable 

 compression is indicated by its comparative hardness and 

 density; in both properties there is a marked increase as 

 compared with the ordinary coal at a distance from a roll. 

 The following figures afford some idea of the degree of the 

 compression, while the miners' experience indicates the 

 superior hardness : 



Specific gravity of coal from crest of a roll ... 1*39 

 Ditto, ditto, 20 yards from the nearest roll... 1*15 



Compression 2&f 



It should he remarked that the popular idea — that a good 

 deal of coal is lost on account of the roll— is incorrect. 

 Some of the coal is compressed, some is rolled out to the 

 sides, but material loss there is none. The floor rocks also 

 show evidence of powerful compression in the roll. The 

 shale is brightly polished and slickensided. 



