Structural geology . layers contrasted in jasper. The broken darklayers are chert, the li^ht layers are secondary iron oxide. Folds may be formed by means of minute displacements alongnumerous joints and faults. Folds in brittle quartzite beds arecommonly of this type. There is no interior deformation of the fault and joint blocks,and there is no thickening or thinning of the beds as a whole. Thetop and bottom of a bed are parallel throughout. The fold is ofthe parallel type. The curvature of the beds so folded is not FOLDS OF FRACTURE AND FLOW CONTRASTED 109 the same through any considerable
Structural geology . layers contrasted in jasper. The broken darklayers are chert, the li^ht layers are secondary iron oxide. Folds may be formed by means of minute displacements alongnumerous joints and faults. Folds in brittle quartzite beds arecommonly of this type. There is no interior deformation of the fault and joint blocks,and there is no thickening or thinning of the beds as a whole. Thetop and bottom of a bed are parallel throughout. The fold is ofthe parallel type. The curvature of the beds so folded is not FOLDS OF FRACTURE AND FLOW CONTRASTED 109 the same through any considerable vertical distance. A muchfolded bed may be replaced above or below, usually below, by amuch less folded bed or one which is deformed almost none at all;in other words, there is a dying out of the fold. Disappearance offolds with depth is discussed on pages 124-127. The difference inthe shortening of the adjacent strata involves slipping between thebeds. This slipping is really of the nature of faulting, although the. Fig. 53. Folding of brittle and soft layers contrasted in jaspercracks in the brittle layers. Note the tension movements are not ordinarily described as faults, on account oftaking place parallel to the bedding. In the zone of fracture rocks are relatively competent; theydo not crumple by interior adjustment; the folds therefore tend tobe simple and open. In the zone of flowage rocks are folded by interior adjustment ofall parts of the mass with development of cleavage. Beds arethickened and thinned. No part of the rock mass is competent towithstand the load without interior adjustment and result is a much more composite or complex folding. Thebed thereby becomes thickened and strengthened, enabling it to
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