C— GEOLOGY. 71 



and especially is this the case with the rising folds. The depressions, 

 however, are different. Our colliers apply the term ' swilly,' or some- 

 times ' swamp,' to shallow, trough-like inflections of a seam. These 

 vary in depth from 2 or 3 feet up to as much as 50 or 60 feet; they 

 vary greatly in breadth, but, so far as I have seen them, they are all 

 steep-sided, perhaps 20° to 40°. Their linear extension ranges within 

 wide limits ; there is one at Rockingham which is known to extend for 

 more than a mile. It has a breadth of 6 chains (132 yards) and a depth 

 of 26 feet. A yet larger one traverses the whole extent of a colliery in 

 Nottinghamshire. The evidence that these depressions were produced 

 contemporaneously is in many cases decisive. Not only is the coal 

 conspicuously thicker in a swilly than the normal, but the infilling is 

 frequently of a different character from the normal roof material. In 

 some cases it carries a patch of cannel; in others, while the normal 

 roof passes over the swilly without bending, between coal and roof, a 

 muddy deposit levels up the hollow Swillies are peculiar to a given 

 seam, and I have learned of only one case in which more than one seam 

 is affected by the same fold; but here it is also noted tliat, as in all 

 wash-outs, the swillies are anterior to, and are thrown by, the 

 faulting. 



It seems probable that the isolated patches of cannel by which some 

 coal-seams are surmounted may, in other cases than those of swillies, 

 lie in hollows produced by earthquake deformation ; and Puller's picture 

 of Reelfoot Lake tempts the reflection that upon its floor the maceration 

 of peat into cannel substance may now be proceeding. If it were not so 

 distant I would fain test it with a few probings. 



The ' Cleat ' or ' Slynes ' of Coal. 



One feature of coal-seams I must discuss before I conclude, though 

 it will not at first appear clear that it can be brought within the title 

 of this address — I allude to the cleavage or cleat or slynes of coal. If 

 we look at a piece of coal this cleavage is very conspicuous, for, lying 

 at right angles with the bedding, it gives the straight sides to the 

 fraigment. It is obviously not, like the cleavage O'f slate, a texture, but 

 it is a series of well-developed joints varying in their individual vertical 

 extension, some being restricted to a single layer of bright coal, and 

 here and there one traversing bright and dull and fusain alike. A 

 thick layer of fusain very commonly interrupts most of the cleat planes 

 that have affected the other materials, and it is seen in such instances 

 that chips of woody texture lie quite across the ineffective cleat. 



It is a vital element in the cleat problem that it is as well developed 

 and as definite in direction in a flake of bright coal the i-ooth of an inch 

 in thickness as in a tree-trunk. While I was preparing this address I 

 procured a slab of shale from the bed underlying the uppermost bed 

 of the Millstone Grit. It bore numerous imprints of goniatites and a 

 leaf of Cordaites, which, in its present condition of bright coal, varies 

 in thickness from about /oth down to xstt*^^ ^^ ^^ ii^^^"^ ^" thickness. 

 It is traversed by an even and regular cleat at intervals of about j-nrs^'h 

 of an inch, disposed at an angle of about 35° to the length of the leaf. 



