[Illustration: A Yorkshire Pot-Hole: showing the Effects which can be Produced in Limestone by Underground Water The immense depth may be better realised by comparing the pot-hole with the Nelson Monument, which is 162 feet in height.] 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 rearrangement 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 fragments of the bony structure. Similarly the archæologists 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, how the forces of nature are at work to-day, can attempt to describe the conditions under which rocks were laid down before man ever trod them.