DEFORMATION BY FOLDING 795 



on mashing. It is often not apparent in fresh rocks, but, as in the 

 case of slaty cleavage, is developed on exposure to the atmosphere. 

 17. Gneissoid Structure. This structure, like schistosity, re- 

 sults from intense mashing and recrystallization of rocks subjected 

 to symphrattic metamorphism. Gneissoid structure is essentially 

 characterized by banding, the bands being of unlike composition. 

 The mineral particles also interlock so that the cleavage is much less 

 perfect than in schists. Usually it is parallel to the banding, but 

 this is not always the case. Gneissoid structure may be developed 

 in igneous and in clastic rocks. In the former case we have granite- 

 gneisses, diortite-gneisses, etc., in the latter arenyte or sandstone 

 gneisses, rudyte gneisses or conglomerate gneisses, etc. 



e. Deformations Due to Folding, and to Folding and Erosion. 



18. Folding. Rock folds are among the most conspicuous and 

 easily recognized tectonic features. They vary greatly in magni- 

 tude, from the minute wrinkles formed in the axes of larger folds, 

 to those whose limbs are many miles apart. In regions of erosion 

 generally only a part of the fold is found, the folded strata having 

 been truncated and cut down until only portions of the limbs re- 

 main. In this way the appearance of tilted strata is produced, these 

 tilted strata being, however, only the remnants of great folds. 



The form of folds is very variable, but it has been possible to 

 select a number of distinct types, of which the others are variants. 

 These distinct types are (a) anticline, (b) synclines, (c) isoclines, 

 (d) fan folds, and (e) monoclines. Compound anticlines are 

 anticlinoria and compound synclines are synclinoria. 



(a) Anticlines, In this type the sides or limhs of the fold 

 typically slope away from the plane of the axis on either side. The 

 sloping portions are known as the limbs of the anticline, and the 

 amount of slope as compared with the horizontal is the angle of dip. 

 All folds are wrinkles in the earth's crust, and if followed far 

 enough along the axis they will die out. The amount of inclination 

 of the axis of the fold, also measured from the horizontal, is called 

 the pitch. Every anticlinal axis pitches in two directions, i. e., to- 

 ward the two ends of the folds. A short anticline in which the two 

 axes are of approximately equal length is a dome. 



(b) Synclines. When the limbs of the fold dip toward the axis 

 a trough fold or syncline is produced, the axis of which pitches to- 

 ward the center of the fold. A short syncline, in which the two 

 axes are of nearly equal length, is a basin. 



