1^2 
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 preserves 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 wdde and extensive, even up to several hundred 
yards. In the twisted shake the strata are waved and disordered 
in several directions, dipping in a confused manner as if m various 
segments of a circle. 
Sometimes a bason is formed in the strata; that is, the horizontal 
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 contains 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 
observed, 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 . 
