II. GEOGNOSY. 



OF FORMATIONS. 



55. It has been a disputed question, whether the solid crust 

 of the globe were formed at a given epoch, and called into 

 existence in the state in which we find it, or whether it have 

 been the result of a succession of actions, whose influence is 

 continued down to the present day. The former idea has 

 been, at last, wholly abandoned, and we shall, as we pro- 

 ceed, develop innumerable facts, by which it will be shown, 

 that the present state of the crust of the globe has been at- 

 tained by slow and gradual steps, under the action of causes, 

 which, if perhaps less intense, are still to be perceived slowly 

 changing the surface of the earth. The period which has 

 thus been occupied in bringing the land into its present state, 

 cannot be defined in years, or even centuries, but may 

 readily be divided into geological epochs. 



56. The term formation is, in some degree, vague ; in its 

 most limited sense it is confined to rocks of identical charac- 

 ter, or in which some one rock predominates. In a wider 

 sense, it includes all the rocks which can be assigned to a 

 single geological epoch. 



57. When rocks are stratified, or when their character is 

 such as leaves no doubt that they have been deposited either 

 chemically, or mechanically, from water, we can judge with- 

 out difficulty of their relative date, by the mere order of their 

 superposition. It being evident, in the latter case, that the 

 lower rock must have been first deposited, and probable that 

 such was the fact, even where the cause of the formation has 



