CHANGES OF POSITION AND LEVEL. 719 



The earth's crust has inequalities of thickness and texture dis- 

 tributed probably in large areas ; and therefore, while conforming 

 to the principle exemplified, it should present peculiarities in the 

 arrangement of the effects dependent on the distribution of these 

 diverse areas. 



The cause is one in which the whole sphere has acted as a unit ; 

 and its effects must, therefore, be coextensive with its surface, but 

 differing in different parts. 



The cause, moreover, must have continued in constant action as 

 long as cooling continued ; for cooling, however slight, implies con- 

 traction and gradually augmenting tension, and an ultimate yield- 

 ing when the tension is too great to be longer resisted. 



The direction in which a force of this kind acts is approximately 

 horizontal within the crust, the contraction creating a strain or 

 pressure between every adjoining part of it ; and, wherever the crust 

 should yield under the tension, some parts would be, as a primary 

 effect, drawn downward, and others, as a secondary effect, pushed 

 upward, — the latter rising through the lateral pressure or pushing 

 action of the subsiding portion ; and fracture would succeed frac- 

 ture, and thus one mass rise over another, or else fold would suc- 

 ceed fold in parallel ranges, or both fractures and folds would take 

 place together. The results would vary with the nature of the 

 crust in the part raised, its condition at the time as to the presence 

 of moisture and heat, and the kind of action in the moving power. 



The natural position of the axes of the plications is at right 

 angles to the direction of the pressure. But if the force is not 

 uniform, and increases in one direction or the other, — a very pro- 

 bcible condition, — the plications would show it in variations as to 

 number, height, and position. Curves might thus result either in 

 the axes of the plications, or in the line of maximum effect over a 

 plicated region. 



2. Examples of effects under the cause last mentioned, with 

 additional explanations. 



1. The effects universal over the globe. — Since the developments with 

 regard to the structure of the Appalachians, made in the course of 

 the geological surveys of the States of Pennsylvania and Virginia, 

 were first published by the Professors Rogers in 1842, it has been 

 found that nearly all inclined strata over the globe are actually 

 portions of plicated strata ; and the general principles mentioned 

 on pages 403-407 (which should here be reviewed by the reader), 

 although deductions from the special case of the Appalachians, 

 are, in fact, universal principles. There is evidence everywhere that 



