MAPPING. 37 



valleys the higher and older enclosing the remains of 

 extinct animals and of paloeolithic man, each succeeding 

 lower terrace more nearly approaching to the existing 

 state of things, and the last deposits having been formed 

 within quite recent periods. 



These old valley gravels are now reduced to mere 

 patches in comparison with their former extension, but 

 from their consistency are much more easily mapped 

 than are the glacial drifts so also are the latest deposits 

 of all, such as brick- earth, gravel, and sand, at about the 

 same level as the rivers. The recent beds are frequently 

 found first on one side, then on the other of existing 

 streams, the present channel running along one edge, 

 then cutting across to follow for a distance the other 

 side, as shown by the stippled area in the frontispiece. 

 The lines of alluvium or marsh-land are as we have seen 

 (p. 23) readily drawn, and it is a good plan in making a 

 geological survey for this deposit to be the first mapped. 

 One thus gets a chance of seeing sections in the lowest 

 beds, and obtains also an idea of the physical geography, 

 or at all events of the valley-system, of the district. 



