TRELIMINARY CONSIDERATIONS. 7 



nnd that this is not their original position is shown by the 

 posture of the stems of trees, once erect, but now inclined 

 with the beds. This leads to a consideration very important 

 with reference to our present subject; namely, that as our con- 

 tinents are mostly made up of beds deposited under water and 

 afterwards elevated, these beds have in this process experienced 

 such disturbances that they rarely retain their horizontal posi- 

 tion, but are tilted at various angles. When we follow such 

 inclined strata over large areas, we find that they undulate in 

 great waves or folds, forming what are called anticlinal and 

 synclinal lines, and that the irregularities of the surface of the 

 land depend to a great extent on these undulations, along with 

 the projection of hard beds whose edges protrude at the sur- 

 face. In point of fact, as shown in Fig. 5, mountain ranges 

 depend on these crumplings of the earth's crust; and the 



Fig. 5. — Ideal section of the Apalachian Mountains, showing folding of the earth' 



crust. 



<r, Anticlinal axes. /', Overturned stnita. c, Synclinalb. (f, Unconformable beds. 



primary cause of these is probably the shrinkage of the mass 

 of the earth owing to contraction in cooling. When the dis- 

 turbances of beds are extreme, they often cause intricacies of 

 structure difficult to unravel ; but when of moderate extent 

 they very much aid us in penetrating below the surface, for 

 we can often see a great thickness of beds rising one from 

 beneath another, and can thus know by mere superficial ex- 

 amination the structure of the earth to a great depth. It thus 

 happens that geologists reckon the thickness of the stratified 



