6 LAND MAMMALS IN THE WESTERN HEMISPHERE 



it is entirely feasible to establish the mode of formation of 

 almost any rock (aside from certain problematical rocks, which 

 do not enter into our discussion) and to determine whether it 

 was laid down in the sea, or on the land, or in some body of 

 water not directly connected with the sea, such as a lake or 

 river. With the aid of the microscope, it is easy to discriminate 

 volcanic material from the ordinary water-borne and wind- 

 borne sediments and, in the case of the rocks which have 

 solidified from the molten state, to distinguish those masses 

 which have cooled upon the surface from those which have 

 solidified deep within the earth. 



While the nature and mode of formation of the rocks may 

 thus be postulated, it will be needful to explain at some length 

 the character of the evidence from which the history of the 

 earth may be deciphered. First of all, must be made clear the 

 method by which the events of the earth's history may be 

 arranged in chronological order, for a history without chronology 

 is unintelligible. The events which are most significant for 

 our purpose are recorded in the rocks which are called stratified, 

 bedded or sedimentary, synonymous terms. Such rocks were 

 made mostly from the debris of older rocks, in the form of 

 gravel, sand or mud, and were laid down under water, or, less 

 extensively, spread by the action of the wind upon a land- 

 surface. Important members of this group of rocks are those 

 formed, more or less completely, from the finer fragments 

 given out in volcanic eruptions, carried and sorted by the wind 

 and finally deposited, it may be at great distances from their 

 point of origin, upon a land-surface, or on the bottom of 

 some body of water. Stratified or bedded rocks, as their 

 name implies, are divided into more or less parallel layers or 

 beds, which may be many feet or only a minute fraction of an 

 inch in thickness. Such a division means a pause in the pro- 

 cess of deposition or a change in the character of the material 

 deposited over a given area. Owing to the operation of 

 gravity, the layers of sediment are spread out in a horizontal 



