1006 A TREATISE ON METAMORPHISM. 



are adjusted to their new positions mainly by differential movements 

 between the separated parts. The so-called folds in the zone of fracture 

 are largely the result of numerous parallel joint fractures across the strata 

 with small angular displacements at the joints, giving each block a slightly 

 different position from those on either side, and thus as a whole making 

 joint folds. For instance, the folds of the rigid rocks in the Alleghenies 

 are not in the main true flexures, but a series of slightly displaced blocks. 



It is shown on pages 189-190 that, making all the assumptions in 

 favor of as great a thickness as possible, the maximum thickness of the zone 

 of fracture cannot be- greater than 10,000 or 12,000 meters. In many 

 cases, even for the strongest rocks, deformation frequently takes place by 

 flowage at depths not greater than one-third to one-half of 10,000 meters. 

 For the weaker rocks deformation by flowage may take place at very 

 moderate depths. If the conclusions above given be correct, it follows that 

 all open fissures must disappear at moderate depths, and that the maximum 

 depth at which they can exist is the depth of the zone of fracture for the 

 strongest rocks. Illustrations of the disappearance of fissures with depth 

 are known at various places. In the g*old belt of the Sierra Nevada, as 

 Lindgren says, it is "an incontestable fact that many small veins close 

 up in depth." 



The dying out of fissures below is insisted upon in order to exclude the 

 hypothesis of filling of fissures from the bottom. If fissures gradually 

 decrease in size and finally die out, the streams which make their way into 

 the fissure must enter from the sides or from above. For further develop- 

 ment of this point see pages 1069-1072. 



OPENINGS OF ZONE OF FRACTURE. 



It has been shown that the zone of fracture is characterized by open- 

 ings. It will be seen that the nature of the openings in the rocks is of the 

 greatest importance in the formation of ore deposits. It is therefore 

 necessary to summai-ize the material given in Chapter III upon this subject. 



It is there shown that the openings of rocks deserve consideration from 

 three points of view: The form and continuity of the openings; the size of 

 the openings; and the volume of the openings, or the pore space. 



