278 



MATERIALS AND THEIR ARRANGEMENT 



Fig. 262. Map record of dip and strike showing 

 complex structure. 



h 



another, the structure of the rock, as shown in Figs. 262 and 263, 



may be worked out, even though the surface is flat. 



Much the larger portion of the earth's surface is occupied by 



beds that depart but little from their original horizontal attitude, 

 v, but in many moun- 



y y tainous regions the 



t beds have suffered 



bending, folding, 

 crumpling, and crush- 

 ing, in various degrees. 

 Distortion is on the 

 whole most consider- 

 able in the most an- 

 cient rocks. Distortion 

 is assigned chiefly to 

 lateral thrust arising 

 from the shrinkage of 

 the earth, as explained 

 in Chapter VIII. 



Complicated struc- 

 tures may be very dif- 

 ficult of interpreta- 

 tion. Thus overturned 



Fig. 263. Cross-section of Fig. 262. along the line ab. , ,, ., , 



folds reverse the order 



of the strata in the under limb of the fold (Fig. 253). After such 

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

 their relations have become obscure, the beds are likely to be in- 

 terpreted as though they lay in natural 

 order. Thus Fig. 264 might represent a 

 simple monoclinal structure, or any one of 

 the complex structures shown in Figs. 265, 

 266, or 267, so far as dip and strike show. 



Joints. The surface rocks of the earth are almost universally 

 traversed by deep cracks called joints (Figs. 2 and 268). In most 

 regions there are at least two systems of joints, the members of 

 each system being roughly parallel, 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 may be three, four, 

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

 in exceptional cases, inches, or even a fraction of one inch. 



Fig. 264. Diagram rep- 

 resenting either isoclinal 

 or monoclinal structure. 



