718 DYNAMICAL GEOLOGY. 



have happened. In neither case could any permanent elevations 

 of the surface, or any plications of the crust, have arisen from 

 such a cause. The rocks show that these plications have been ex- 

 tremely slow in progress (p. 411), and not a result of any paroxys- 

 mal action in forces, above, within, or below the crust. 



8. Change of temperature producing expansion and contraction. — Change 

 of temperature may have acted in two ways : — 



(a.) By simple expansion and contraction of some limited region 

 within the earth's crust, as when heated from proximity to some 

 volcanic or other igneous source of heat. The effect would be (1) 

 a rising of the surface, with whatever might be upon it, with the 

 expansion ; (2) a sinking of the same with the subsequent contrac- 

 tion on cooling ; (3) a lateral action or pressure on an adjoining 

 region with the expansion (since it would tend to take place late- 

 rally as well as vertically), producing horizontal movements or dis- 

 placements of small amount; (4) in some cases, on contraction, an 

 opening of cracks or fissures, either thickly over the whole sur- 

 face (as in the case of basalt and trap when divided into polygonal 

 columns), or more distant and of greater width. The oscillations 

 of level in the temple of Jupiter Serapis and along the adjoining 

 coast have been explained by this method. 



(b.) By contraction going on within the earth's interior beneath 

 its solidified crust. The fact that this cause has acted in the earth's 

 past is beyond question if the globe was once in a fused state, as is 

 generally supposed by geologists. Since the crust when formed 

 would have had the size which the globe then had, all subsequent 

 cooling, as it would tend to diminish the interior, would bring a 

 slowly-increasing strain upon it, and, unable to accommodate itself 

 to the changing size by any process of shrinkage, it must do it 

 either by fractures or plications, or both. 



The effect, in a melted spheroid, of cooling more rapidly at the 

 surface than within, is illustrated in glass in a Prince Eupert's drop. 

 The pressure of particle against particle over the whole exterior is 

 so great from the interior contraction that the removal of a portion 

 of the surface-layer by a slight scratch of a file destroys the equi- 

 librium, and causes it to break instantly, and almost explosively, 

 to fragments. Another familiar example of contraction beneath 

 an exterior coat is seen in a drying apple. The exterior, in this 

 case flexible, gradually becomes wrinkled from the diminution of 

 size within ; and the wrinkling covers the whole surface alike, unless 

 some part be protected by resin or otherwise, — in which case the 

 largest wrinkles would be those about the border of the protected 

 portion. 



