STRATIFIED ROCKS. 
91 
have merely been rubbed in one direction, which is far from 
being a constant phenomenon. Not only are some sets of 
strise not parallel to others, but the clay and rubbish be¬ 
tween the walls, when squeezed or rubbed, have been streak¬ 
ed in different directions, the grooves which the harder min¬ 
erals have impressed on the softer being frequently curved 
and irregular. 
The usual absence of protruding masses of rock forming 
precipices or ridges along the lines of great faults has al¬ 
ready been alluded to in explaining Fig. 76, p. 89, and the 
same remarkable fact is well exemplified in every coal-field 
which has been extensively worked. It is in such districts 
that the former relation of the beds which have been shifted 
is determinable with great accuracy. Thus in the coal-field 
of Ashby de la Zouch, in Leicestershire (see Fig. 77), a fault 
Faults and denuded coal-strata, Ashby de la Zouch. (Mammatt.) 
occurs, on one side of which the coal-beds a, 5, c, d must once 
have risen to the height of 500 feet above the corresponding 
beds on the other side. But the uplifted strata do not stand 
up 500 feet above the general surface ; on the contrary, the 
outline of the country, as expressed by the line z is uni¬ 
formly undulating, without any break, and the mass indi¬ 
cated by the dotted outline must have been washed away.^ 
The student may refer to Mr. Hull’s measurement of faults, 
observed in. the Lancashire coal-field, where the vertical dis¬ 
placement has amounted to thousands of feet, and yet where 
all the superficial inequalities which must have resulted from 
such movements have been obliterated by subsequent denu¬ 
dation. In the same memoir proofs are afforded of there 
having been two periods of vertical movement in the same 
fault—one, for example, before, and another after, the Trias- 
sic epoch.f 
The shifting of the beds by faults is often intimately con¬ 
nected with those same foldings which constitute the anti- 
* See Mammatt’s Geological Facts, etc., p. 90 and plate, 
t Hull, Quart. Geol. Journ., vol. xxiv., p. 318. 1868. 
