800 



DYNAMICAL GEOLOGY. 



of the width and length of the block ; and this lateral movement or 

 flow had bulged the sides much more at bottom than at top, and most 

 about the middle. At bottom the block was increased l-25th in 

 width and ]-50th in length. The block had been made of plates 

 of iron welded together, and these were bent downward as the punch 

 passed in, the lower ones the least ; and Fig. 1151 shows the appear- 

 ance of the surface, after polishing and etching it with acids, of a sec- 

 tion through the middle, when the punch had entered 1^ inches and 

 the core projected an eighth of an inch. The molecular flow is essen- 

 tially like that of pitch or any other fluid. 



Strata, when they were folded, were generally (1) those of sedimen- 

 tary formations ; (2) only partially consolidated, limestones excepted ; 

 (3) containing more or less moisture and having the cohesion dimin- 

 ished thereby ; and, (4) as the movement proceeded, they were receiv- 

 ing heat through the friction, which, if low in degree, made siliceous 

 solutions and so further diminished friction ; and, if high, produced 

 superheated steam with a general softening of the flexing masses. 



2. Forms of Flexures. — The flexures undergone by strata are not 

 easily reproduced by experiment in the small way, because the ele- 

 ment of weight is one of chief importance. The weight is very great 

 if only 20,000 feet of rock are undergoing flexure together. It influ- 

 ences largely the shapes of flexures, however the disturbing force be 

 applied, and increases the resistance from the underneath terranes that 

 do not participate in the movement. 



The contraction of a stretched strip of caoutchouc, overlaid by ad- 

 hering layers of clay, has been used to exemplify the flexures in the 

 earth's supercrust over the contracting crust. Flexed and steeply up- 

 turned beds and gaping fractures are obtained ; but the result fails of 

 imitating nature in two respects; (1) the flexures have not the un- 

 equal-sided character which pushing from one direction produces ; and 

 (2) they are a continuously progressing effect of the contraction, when 

 mountain-making over the earth has occurred at widely separated 

 epochs. 



The effects of flexure are modified by differences in the beds as to 

 texture — some being made to break, when others stretch or become 

 compressed ; also by inequalities as to amount of moisture and heat 

 present, and in their conductivity of heat — determining differences as 

 to facility of movement. 



Shaly or slaty strata, when raised into nearly vertical position, 

 would be liable to have the layers crumpled or finely plaited by sim- 

 ple weight, as well as by the pressure that caused the uplift, while 

 sand-beds might settle without leaving any evidence of it 



