CAUSES OF FOLDING AND FAULTING 255 



sight, to suppose that the direction of the force was vertically up- 

 ward, acting with maximum intensity beneath the anticlines and 

 with minimum intensity beneath the synclines. But such an ex- 

 planation could only apply to open, symmetrical, and simple folds, 

 and even in these cases, is not satisfactory. Folded strata must 

 either occupy less space transversely than they did before folding, 

 or else they must have been stretched and made much thinner, but 

 a comparison of continuous beds, in the flexed and horizontal 

 parts of their course, shows no such thinning. Again, such an ex- 

 planation is obviously insufficient to account for closed, inclined, 

 and inverted folds, for contortions and plications, and for flexures 

 of different orders, one within another. 



If the folding force did not act vertically, it must have acted 

 horizontally, and this is the explanation now almost universally 

 accepted. A horizontally acting force would compress and crumple 

 up the beds, producing different types of flexure in accordance 

 with varying circumstances. Furthermore, the microscopic study 

 of folded rocks shows that they have actually been compressed 

 and mashed and the minutest plications are visible only under the 

 microscope. 



Assuming, then, that the folding force was one of compression 

 and acted horizontally, we have next to consider the circumstances 

 which modify the result, producing now one form of flexure or 

 fracture, now another. Such modifying circumstances are the 

 depth to which a given stratum is buried, its thickness and rigidity, 

 the character of the beds which are above and below it, and the 

 intensity and rapidity with which the flexing force is applied. 

 When in a mountain region one sees the manner in which vast 

 masses of rigid strata are folded and crumpled like so many sheets 

 of paper, one perceives the enormous power which is involved in 

 these operations and the gradual, steady way in which that power 

 must have been exerted. When strata are buried under a suffi- 

 cient depth of overlying rock to crush them, they become virtually 

 plastic and yield to the compressing force by flowing without fract- 

 ure. At such relatively great depths cavities cannot exist, and if 

 the compressed rock should be broken by the compression, the 



