CHAPTER XVII 



FAMILIES OF ROCKS AND THEIR 

 DESCENDANTS 



THUS far, in accordance with the principles of the 

 great geologists from Sir Charles Lyell onwards, we 

 have tried to disclose the history of the earth's 

 crust by observing the processes which are going on 

 to-day under our eyes. That is not, however, the only 

 way in which history has to be written. The documents 

 on which history rests are often lamentably incomplete. 

 The records have great gaps in them, and very often the 

 gaps have to be filled by that exercise of the imagination 

 which Bishop Creighton once described as the rearrange- 

 ment of facts. We shall later in this book show how 

 naturalists can reconstruct the skeleton and even the 

 general appearance of an animal which for ages has not 

 been seen alive on the earth, from a consideration of frag- 

 ments of the bony structure. Similarly the archaeolo- 

 gists who inquire into the history of forgotten peoples 

 can picture to us their lives and habits and manners from 

 a consideration of the fragmentary weapons and pottery 

 and architecture which they left in their buried cities; 

 and similarly the geologist, knowing, or partly knowing, 



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