94 
ELEMENTS OF GEOLOGY. 
for example, Mr. Hull has pointed out that three principal 
lines of disturbance, all later than the Carboniferous period, 
have affected the stratified rocks of Lancashire. The first 
of these, having an E.IST.E. direction, took place at the close 
of the Carboniterous period. The next, running north and 
south, at the close of the Permian, and the third, having a 
N.N.W. direction, at the close of the Jurassic period."^ 
Unconformability of Strata.—Strata are said to be uncon- 
formable when one series is so placed over another that the 
planes of the superior repose on the edges of the inferior 
(see Fig. 78). In this case it is evident that a period had 
elapsed between the production of the two sets of strata. 
Unconformable junction of old red sandstone and Silurian schist at the Siccar 
Point, near St. Abb’s Head, Berwickshire. 
and that, during this interval, the older series had been tilt¬ 
ed and disturbed. Afterwards the upper series was thrown 
down in horizontal strata upon it. If these superior beds, 
rf. Fig. 78, are also inclined, it is plain that the lower 
strata, a, a, have been twice displaced; first, before the dep¬ 
osition of the newer beds, d, and a second time when 
these same strata were upraised out of the sea, and thrown 
slightly out of the horizontal position. 
It often happens that in the interval between the deposi¬ 
tion of two sets of unconformable strata, the inferior rock 
ig. 79. 
Junction of unconformaoie strata near Mons, in Belgium. 
has not only been denuded, but drilled by perforating shells. 
Thus, for example, at AutrejDpe and Gusigny, near Mons, 
beds of an ancient (primary or palseozoic) limestone, highly 
inclined, and often bent, are covered with horizontal strata 
* Edward Hull, Quart. Geol. Journ., vol. xxiv., p. 323. 
