140 STRUCTURAL AND FIELD GEOLOGY 
this structure two unsymmetrical synclines approach each 
other from opposite directions, while in the intervening 
space the strata are arched into a great anticline” ) Ime 
crown of the anticline has invariably disappeared, so that the 
truncated strata are seen to dip in from both sides towards 
the axial plane. Since the beds within the anticline are 
much compressed below while they open out above, they 
present the appearance known as fan-shaped structure (see 
Fig. 30). 


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“Bae. 30.—SECTION ACROSS MOUNT BLANC, SHOWING 
FAN-SHAPED STRUCTURE. 
All the several kinds of unsymmetrical folds described in 
the preceding paragraph occur in regions which have been 
subjected to some dominant movement of the crust—either 
of elevation or depression. When a broad zone has bulged 
up under lateral pressure to form a mountain chain, we have, 
as in the Alps, one great arch composed of numerous sub- 
ordinate wrinkles or minor folds and flexures. A complex 
arch of this kind is termed an Anticlinorium (see Fig. 31). 
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FIG. 31.—DIAGRAM OF AN ANTICLINORIUM. 
If the arch be simple—a broad anticlinal fold with no 
conspicuous wrinklings or flexures—it is known as a 
Geanticline. The converse structure, resulting from the 
depression of a broad zone, is termed a Synclinorium, when 
