CLASTIC ROCKS 121 



series of neighbouring sections. But here and there the complete 

 structures may be seen in long cuttings or large quarries (Fig. 29). 



Care should be taken to discriminate between strata which owe 

 their inclination to .the elevation and folding of sediments that 

 were originally horizontal and these beds which were deposited on 

 inclined surfaces and built up irregularly under the action of cur- 

 rents of water. The latter is false bedding, the former is known 

 as inclined bedding (contrast Fig. 29 with Fig. 17). Inclined beds 

 can be in imagination stretched out till they are flat again, while 

 false bedding could not be thus straightened out, the beds having 

 always been irregular in inclination and wedge-like or lens-like in 

 shape. Moreover, in false bedding the slope of the layers does 

 not exceed the "angle of rest " of the material of which they 

 are made, while " inclined beds " may lie at any angle, because 

 they have been tilted, in most cases after consolidation. The one 

 indicates hurried and irregular deposition, the other calm and 

 quiet deposition on a flat area and the subsequent tilting and 

 folding of flat strata. 



The earth's crust is therefore undergoing not only destruction 

 by denudation, but renewal by deposition and earth movement. 

 The debris swept off by streams and the sea is being conserved 

 in areas of deposition, and eventually it is lifted again to form 

 new land. In the process great changes in position, height, and 

 nature of the land are being brought about. Thus none of the 

 features of the earth are constant in nature or position for any 

 great length of time. Present denuding agencies are capable of 

 destroying all the land of the globe in three millions years, and, as 

 the land has not been destroyed, deposition and elevation must 

 have been capable of renewing it within the same time. The one 

 process, that of degradation, is the work of gravitation, dragging 

 all things to a lower level ; the other process must be attributed 

 either to the activity of the interior of the earth or to external 

 energy such as the attractive forces of the sun and moon. These 

 are in constant antagonism to one another, and their interaction 

 has prepared and maintained the earth's surface in a condition 

 suitable for the continued existence of life on it. 



The study of different stratified rocks proves that those parts 

 of the earth's surface in which they occur must have passed 



