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 impossible that the whole crust can be involved in folds of such small amplitude. The crust is, however, subject to flexures of its own, 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. The great thickness of sediments which form the Appalachian Mountains (exceeding 25,000 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 the rate of deposition, that almost the entire thickness was accumulated in shallow water, as is indicated by the character of the rocks themselves. Geanticlines are less easy to detect, but there is evidence to show that they do occur on an equally great scale.

Folds may be classified either in accordance with the relation which their opposite limbs bear to each other, or with reference to the degree of compression to which they have been subjected. Using the first method, we may distinguish the following varieties.

Fig. 158. - 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.