172 



turns foul, and is mixed with stone or clay, for a greater or less 

 extent; this is termed A TROUBLE. In A SHAKE the strata are not 

 entirely broken and separated by a fissure or chasm, but are only 

 shaken and cracked, as it were, and thrown into confusion ; the 

 coal being not altogether lost, but rendered useless ; being 

 soft, tender, and shattery. The waving SHAKE is that in which, 

 although there is a waving up and down of the seam, yet it pre- 

 serves nearly the same stretch and bearing. Some of these do not 

 continue long before the coal comes in, and is continued again 

 with its usual regularity ; but others prove very wide and extensive, 

 even up to several hundred yards. In the twisted SHAKE the strata 

 are waved and disordered in several directions, dipping in a con- 

 fused manner as if in various segments of a circle. 



Sometimes A BASON is formed in the strata; that is, the hori- 

 zontal strata, to a greater or less extent, are observed to fall lowest 

 in the middle, and rise gradually to the outskirts, all round, in the 

 form of a piece of low hollow ground, in a meadow, which con- 

 tains a lake of stagnant water in a rainy season. These basons, as 

 well as the various other positions of the strata, are found upon 

 various scales, from a quarter of a mile to a mile in diameter*. 



Sometimes a waving of the strata into furrows and ridges is ob- 

 served, the same seam of coal rising many times to the surface. 

 Sometimes the stratum of coal dips and rises, with the surface of 

 the ground ; but this is in general not far continued, and seems to 

 be entirely accidental ; since we shall as often, or indeed much 

 oftener, find it dipping in opposition to the rising of the surface of 

 the ground, and passing, in any direction, without appearing to be 

 at all affected by the figure of the surface. 



Coal fields or coal countries are patches of limited but very dif- 

 ferent dimensions ; the Mid-Lothian coal field is nearly a square of 



* Williams's Mineral Kingdom, vol. i. p. 105. 



