GEOSYXCLINES 



237 



which are characterized by their great width and gentle slope. 

 Such flexures have been named by Dana geanticlines and geo- 

 synclines, to express their importance for the earth as a whole. 



FIG. 95. — Diagrams of folds. (Willis.) 1. Upright or symmetrical open 

 folds. 2. Asymmetrical fold, open. 3. Asymmetrical fold, closed and overturned. 

 4. Symmetrical fold, closed. 5. Closed anticline, overturned. 6. Closed anticline, 

 recumbent. 



The great thickness of sediments which form the Appalachian 

 Mountains (exceeding 30,000 feet) was laid down in an immense 

 geosynclinal trough, which through long ages slowly sank as the 

 sediments accumulated. The rate of subsidence so nearly equalled 



