236 



DISPLACEMENTS OF STRATA 



describing an oval or circle. Similarly, the basin is a syncline 

 with axis reduced to zero, the beds dipping downward from all 

 sides to the bottom of the basin, and the strike forming the edge 

 of the basin. The term basin is used in different senses, and it 

 is necessary to distinguish carefully between a basin of folding and 

 one which has been excavated by erosion. 



It is rare to find a single anticline or syncline occurring by itself; 

 very much more frequently they are found in more or less parallel 

 series, each pair of anticlines connected by a syncline. At one 

 end of the system we may find several axes converging and unit- 

 ing into a single fold, and they all die away sooner or later, the 

 pitch of the folds coinciding with the dip of the beds. 



Anticlinorium and Synclinoriunt. — The system of roughly paral- 

 lel folds which are grouped together may be, when regarded as a 



Fig. 93. — Anticlinorium: section through the Appalachian Mountains. (Willis.) 



whole, either anticlinal, rising up into a great compound arch, or 

 synclinal, depressed into a great compound trough. The former 

 is called an anticlinorium, and the latter a synclinorium. The 

 secondary folds which compose one of these systems may them- 



FlG. 94. — Synclinorium, Mt. Greylock, Massachusetts. (Dale.) 



selves be compound and made up of many subordinate folds, the 

 smallest of which can be detected only with the microscope. 



Geanticline and Geosyncline. — The folds and flexures which 

 we have so far examined are those which affect the strata at the 

 surface or at comparatively moderate depths. It is quite impos- 

 sible that the whole crust can be involved in folds of such small 

 amplitude. The crust is, however, subject to flexures of its own, 



