CURVATURE OF STRATA 135 
in regions of gently inclined strata, we encounter dome- 
shaped and basin-shaped structures. When the strata are 
dome-shaped they are said to have a quaquaversal dip, ze. 
they are inclined outwards in all directions from a common 
point (see Fig. 21). The converse of this structure is seen in 
a centroclinal fold—the beds dipping inwards from all direc- 
tions towards a central point (see Fig. 22). But symmetrical 


— SZ Z 
SSS 8 
as 


FIG. 21.—QUAQUAVERSAL FOLD. FIG. 22.—CENTROCLINAL FOLD. 
a, ground-plan ; b, section along line A—B. a, ground-plan ; 0, section along line A—B. 
or complete quaquaversal and centroclinal folds are of 
somewhat rare occurrence, and may be looked upon as 
accidental modifications of normal anticlinal and synclinal 
folds. 
Normal or Symmetrical Folds.—Strata, as a rule, are 
folded along axes. This is true of the simplest flexures 
(monoclines), and of all the more complex folds to be 
described. The axes or axial planes of normal folds are | 
approximately vertical, and usually extend in straight or | 
gently curving lines. They vary much in length—from a | 
hundred yards or less to many miles. When the strata dip | 

