90 
ELEMENTS OE GEOLOGY. 
the spaces marked in the diagram by dotted lines, and to 
occupy those marked by the continuous lines, then let de¬ 
nudation take place along the line A H, so that the protrud¬ 
ing masses indicated by the fainter lines are swept away— 
a miner, who has not discovered the faults, finding the mass 
a, which we will suppose to be a bed of coal four times re¬ 
peated, might hope to find four beds, workable to an indefi¬ 
nite depth, but first, on arriving at the fault G, he is stopped 
suddenly in his workings, for he comes partly upon the shale 
and partly on the sandstone c; the same result awaits 
him at the fault F, and on reaching E he is again stopped 
by a wall composed of the rock d. 
The very difierent levels at which the separated parts of 
the same strata are found on the difierent sides of the fis¬ 
sure, in some faults, is truly astonishing. One of the most 
celebrated in England is that called the “ninety-fathom 
dike,” in the coal-field of Newcastle. This name has been 
given to it, because the same beds are ninety fathoms (540 
feet) lower on the northern than they are on the southern 
side. The fissure has been filled by a body of sand, which 
is now in the state of sandstone, and is called the dike, which 
is sometimes very narrow, but in other places more than 
twenty yards wide.* The walls of the fissure are scored by 
grooves, such as would have been produced if the broken 
ends of the rock had been rubbed along the plane of the 
fault.f In the Tynedale and Craven faults, in the north of 
England, the vertical displacement is still greater, and the 
fracture has extended in a horizontal direction for a distance 
of thirty miles or more. 
Great Faults the Result of repeated Movements. —It must 
not, however, be supposed that faults generally consist of 
single linear rents; there are usually a number of faults 
springing ofi* from the main one, and sometimes a long strip 
of country seems broken up into fragments by sets of paral¬ 
lel and connecting transverse faults. Oftentimes a great 
line of fault has been repeated, or the movements have been 
continued through successive periods, so that, newer deposits 
having covered the old line of displacement, the strata both 
newer and older have given way along the old line of frac¬ 
ture. Some geologists have considered it necessary to im¬ 
agine that the upward or downward movement in these 
cases w^as accomplished at a single stroke, and not by a se¬ 
ries of sudden but interrupted movements. They appear to 
have derived this idea from a notion that the grooved walls 
* Conybeare and Phillips, Outlines, etc., p. 376. 
t Phillips, Geology, Gardner’s Cyclop., p. 41. 
