68 



LYELL'S ELEMENTS OF GEOLOGY. 



Origin of Curved Strata Zigzag Flexures. 



other books to each end, and force them towards each other. 



The folding of the cloths will exactly imitate those of the bent 



strata. (See Fig. 61.) 



r gj To inquire whe- 



ther the analogous 

 flexures in strata 

 have really been 

 due to similar side- 



way movements, or 

 other exertions of 

 force, would lead 

 me farther into the 

 regions of specula- 

 tion and conjecture 

 than might be consistent with the scope of this elementary 

 work. When the volcanic and granitic rocks are described, 

 it will be seen that some of them have, when melted, been in- 

 jected forcibly into fissures, while others, already in a solid 

 state, have been protruded upwards through the incumbent 

 crust of the earth, by which a great displacement of flexible 

 strata must have been caused. It also appears that cavities are 

 sometimes formed in the interior of the earth, whether by the 

 removal of matter by volcanic action, or by the contraction of 

 argillaceous rocks, or other causes. In this manner pliable beds 

 sinking down, from failure of support, into chasms of less hori- 

 zontal extent, may have become folded and compressed laterally. 

 Such subsidences have been witnessed on a small scale in the 

 undermined ground immediately over coal-pits, from which large 

 quantities of coal and stone had been extracted. 



Fig. 62. 



Zigzag flexures of coal near Mons. 



Between the layers of shale, accompanying coal, we some- 



