CURVATURE OF STRATA 133 
may be (Fig. 17). Again, when two series of strata with 
discordant dips occur in juxtaposition, the one set is said 

FIG. 17.—OUTCROP AND STRIKE. 
to strike at or against the other. The conditions referred to 
are shown in the ground-plan (Fig. 18), where the cause of 
the discordance is the 
presence of a fault (see 
Chap: X1.). 
Survature of 
Strata.—Inclined beds 
are usually, but not 
always, parts of large 
curves or undulations. 
Under certain condi- 
. : FIG. 18.—STRATA STRIKING AT EACH 
tions, as in the case of SOSEE. 
deltas, we may have a 
succession of imbricating and interosculating beds, all the 
members of the series showing a general dip in the direction 
followed by the sediment-transporting current. Further, it is 
obvious that the lower beds of a great succession of strata 
accumulated in a basin-shaped depression, must be more 
or less inclined, according as the floor of the basin shelves 
rapidly or gradually. But with continuous sedimentation the 
inequalities of lake-bottom and sea-floor must eventually be 
obliterated, and the bulk of the deposits come to occupy an 
approximately horizontal position. There is little reason to 
doubt, therefore, that all the great systems of marine 
sedimentary strata were originally for the most part arranged 
in successive horizontal layers and sheets. With such excep- 
tions as those referred to above, the inclined position which 
strata now so frequently occupy must be due to subsequent 
crustal deformation. Strata originally horizontal have been 
thrown into gentle undulations and sharper folds, and the 


