BENEFICIAL EFFECTS OF FAULTS. 543 
would have been inaccessible. (See PI. G5. Fig. 
3. and PI. 66. Fig. 2.) Had the strata of Shale 
and Grit, that alternate with the Coal, been con- 
tinuously united without fracture, the quantity of 
water that would have penetrated from the sur- 
rounding country, into any considerable excava- 
tions that might be made in the porous grit beds, 
would have overcome all power of machinery 
that could profitably be applied to the drainage 
of a mine ; whereas by the simple arrangement 
of a system of Faults, the water is admitted only 
in such quantities as are within control. Thus 
the component strata of a Coal field are divided 
into insulated masses, or sheets of rock, of irre- 
gular form and area, not one of which is conti- 
nuous in the same plane over any very large 
district ; but each is usually separated from its 
next adjacent mass, by a dam of clay, impene- 
trable to water, and filling the fissure produced 
by the fracture which caused the Fault. (See 
PL 66. Fig. 2. and PI. 1. Figs. /,—/, 7.) 
If we suppose a thick sheet of Ice to be broken 
into fragments of irregular area, and these frag- 
ments again united, after receiving a slight de- 
gree of irregular inclination to the plane of the 
original sheet, the reunited fragments of ice will 
represent the appearance of the component por- 
tions of the broken masses, or sheets of Coal 
measures we are describing. The intervening- 
portions of more recent Ice, by which they are 
