
CURVATURE OF STRATA 141 
the great trough is complicated by many subordinate 
foldings, and a Geosyncline when the trough is simple. 

FIG. 32.—DIAGRAM OF A SYNCLINORIUM, 
Contorted Strata.—When strata are so unsymmetrically 
and abundantly folded that it becomes difficult or impossible 
to trace out the individual flexures and crumplings—the 
whole forming an irregular complex of folds—they are said 
to be contorted (see Plates XXX.-XXXII.). Such contorted 
rocks are frequently associated with the several kinds of un- 
symmetrical folds described in the preceding section. In all 
highly folded and corrugated strata, the rocks have obviously 
been subjected to great compression. This is seen in the 
peculiar thinning and thickening undergone by the strata— 
the beds becoming attenuated in the limbs of the folds, and 
swelling out again in the cores of the arches and.troughs 
(see Plates XXXI., XXXTI). It would seem, in fact, as if, 
under compression, the solid rocks had been compelled to 
yield and to behave like plastic bodies. The evidence of 
such shearing and flowing is conspicuous not only in the 
larger folds, but in the smallest crumplings visible to the 
naked eye. Indeed, when the rocks are sliced and examined 
under the microscope, they continue to show precisely the 
same structures. It is thus no exaggeration to say that folds 
vary in dimensions from great flexures measuring hundreds 
of yards across, down to puckerings and crumplings so 
minute that they only become visible under the microscope 
(see Plate V. 4). 
Ifin the case of contorted strata the individual beds and 

