4 /. W. POWELL 



We must now more fully consider the nature of these move- 

 ments. Sometimes upheaval is by anticlinal flexure, where the 

 rocks are lifted along a line of upheaval and caused to dip away 

 on either side in gentle or abrupt slopes which are sometimes 

 beautifully curved ; but such an upheaval often seems to be 

 accompanied by a subsidence on the flanks. Symmetrical anti- 

 clinal flexures are not very common, but often one side slopes 

 gently while the other is abruptly deflected. This abrupt slope 

 is especially subject to rupture, in which case faults are substi- 

 tuted for flexures. Thus a block which dips gently in one direc- 

 tion has its margin, on the side of a fault, displaced as an 

 abrupt escarpment. Blocks formed in this manner often careen 

 upon their edges, so that the strata may become vertically dis- 

 posed or quite overturned where the lower formed strata are 

 found on top. Between careened blocks and flexed blocks no 

 line of demarcation can be drawn : the same block in different 

 parts of its course may be bent or broken, and the flexed blocks 

 themselves be quite overturned. The rocks which are upheaved 

 ■or depressed by faulting and flexing, one or both, are always 

 found to be ruptured in line of the faults or flexures, and also 

 transversely to them. This rupture is often minute, so that the 

 sheets of rock are faulted and jointed and thus found in blocks 

 of varying dimensions, but all very minute as compared with the 

 widely spread formations from which they are broken. Thus 

 the whole system of rocks, of igneous and aqueous origin alike, 

 are broken into blocks by faults and ruptures, and still further 

 divided by planes of deposition, so that the structural crust is a 

 system of fragments sometimes with an area of many yards, 

 other times an area of fractions of inches. When we compare 

 these blocks with the great area of the structural crust we find 

 that it is but an accumulation of blocks that are to the formations 

 what grains of sand are to the blocks. We must now realize 

 that the structural crust nowhere has a continuous coherence ; 

 that faults, joints, and partings render it a vast body of minute 

 and loosely accumulated fragments. All of this upheaval and 

 subsidence with flexures, faults, joints, and partings seem to 



