The system of roughly parallel folds which are grouped together may be, when regarded as a 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 themselves be compound and made up of many subordinate folds, the smallest of which can be detected only with the microscope.

Fig. 157. - Synclinorium, Mt. Greylock, Mass. (Dale).