> Parr VI.| FAULTS. 531° 
fissure. Exceedingly complicated examples occur in some coal- 
fields, where the connected faults become so numerous that no one 
of them deserves to be called the main or leading dislocation. By 
a series of branch faults the effect of a main fault may be neutralized 
or reversed. Suppose, for example, that a main fault at its eastern 
portion throws down 60 fathoms to the north, and that at intervals 
three faults on the same side strike off from it, each having a down- 
— throw of 25 fathoms to the east; the combined effect of these branch 
faults will be to reverse the throw. of the main fault towards its 
western end, and make it a downthrow of 15 fathoms to the south. 
Groups of Faults.—The subsidence or elevation of a large 
mass or block of rock has usually taken place by a combination of 
faults. Detailed maps of coal-fields, such as those published by 
the Geological Survey of Great Britain on a scale of six inches to a 
mile, furnish much instructive material for the study of the way in 
which the crust of the earth has been reticulated by faults. In most 
1a 
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ONE MILE 
B.C. 
Fic. 262.—Map oF PART OF THE SouTH WALES COAL-FIELD, 
_A A, Coal measures; L L, Carboniferous limestone dipping beneath the coal-measures 
as shown by the arrows; a@ a, dip-faults; S, Swansea; M, the Mumbles; 
B. C. Bristol Channel. 
cases, dip-faults are predominant, sometimes to a remarkable extent, 
as in the portion of the South Wales coal-field represented in 
Fig. 262. In other places the dislocations run in all directions so 
as to divide the ground into an irregular network. 
It often happens that, by a succession of parallel and adjcining 
faults, a series of strata is so dislocated that a given stratum which 
may be near the surface on one side is carried down by a series of 
2M 2 
