ROCK-FOLDS 



579 



amount of water held up within it, than of any property possessed 

 by the component grains, since it is manifest that rock holding 

 more water will have a greater amount of solvent to effect the 

 transposition of material than one with less capacity. This 

 feature comes out very clearly in the next example, where the 

 nipping beds have broken along the axis and forced the nipped 

 beds to flow between "jaws " of rock. 



(b) Folds in which stronger beds have caused the weaker ones to 



flow at a greater rate than themselves. — The best example I know 



of illustrating this type of fold lies out on the Karroo to the 



north of the Zwartberg Mountains. Pressure has come upon 



Fig. 5. 



rocks divided into two groups, a lower one composed of close- 

 grained sandstones, and an upper made up of hardened glacial 

 till, the Dwyka Conglomerate. The latter has parted in the axis 

 of the anticline, and the pressure has caused the lower sand- 

 stones to squeeze through a narrow slit in the overlying forma- 

 tion, and they now form an abrupt range of hills rising from 

 the plains, called the Tygerberg. Doubts have been thrown on 

 the anticlinal nature of this inlier, but the two ends plunge 

 normally beneath the Dwyka Conglomerate and clearly demon- 

 strate its nature. If we concede that the capacity for holding 

 solvent water is a measure of a rock's readiness to flow under 

 pressure, then this curious structure is easily explained, for a 



