510 GEOLOGY. 



The simpler, and some rather complex forms of deformation, are shown 

 in the preceding figures, but the folding is sometimes much more com- 

 plex (Fig. 406), the folds sometimes ''fan" (Fig. 407), and the beds of 

 which they are composed are sometimes intricately crumpled (Figs. 408 

 to 410). Among these various phases of deformation there are all 

 gradations and combinations. Overturned folds reverse the order 

 of the strata in the under Hmb of the fold. After such folds have been 

 greatly eroded, so that their outer form is lost and their relations have 

 become obscure, the reversed beds are likely to be interpreted as though 

 they lay in natural order. In such a case as that represented in Fig. 411, 

 a complex structure may be interpreted as a simple one. Thus the 

 strata of Fig. 411 may have the structure shown in Fig. 412, 413, or 414, 

 soiar as dip and strike show. 



Joints. — The surface rocks of the earth are almost universally trav- 

 ersed by deep cracks called joints (Figs. 415, 138 and 140). In most 

 regions there are at least two systems of joints, the crevices of each 

 system being roughly parallel to one another, while those of the two 

 systems, where there are two, are approximately at right angles. In 

 regions of great disturbance, the number of sets of joints is often three, 

 four, or even more. The joints of each set may be many yards apart, 

 or in exceptional cases, but a few inches, or even a fraction of an inch. 



Generally speaking, there are more systems of joints, and more 

 frequent joints in each system, where the rocks are much deformed 

 than where they have been but httle disturbed. In undisturbed rocks 

 the joints approach verticality, but in regions where the rocks have been 

 notably deformed, the joint planes may have any position. Not rarely 

 they simulate bedding planes, especially in igneous and metamorphic 

 rocks (Fig. 416). In the latter case especially, the cleavage due to 

 jointing is often mistaken for bedding. They do not ordinarily show 

 themselves at the surface in regions where there is much mantle rock, 

 but they are readily seen in the faces of cHffs, in quarries, and, in general, 

 wherever rock is exposed (Figs. 138 and 140). Though some of them 

 extend to greater depths than rock has ever been penetrated, joints are, 

 after all, superficial phenomena. They must be Hmited to the zone of 

 fracture, and most of them are probably much more narrowly limited. 

 Joints frequently end at the plane of contact of two sorts of rock. Thus a 

 joint extending down through limestone may end where shale is reached. 



