450 A TREATISE ON METAMORPHISM. 



of the West, especially in the great plateaus, colonies of prairie dogs 

 occupy areas extending each from less than an acre to many acres. 

 Throughout one of their towns are burrows and corresponding hillocks 

 in every direction, at intervals from less than a meter to a few meters. 

 Different varieties of gophers and ground squirrels are very widespread. 

 They occur from the Atlantic to the Pacific. In some areas in the West 

 their burrows and mounds are only less numerous than those of the prairie 

 dogs in the dog towns. The mole is scarcely less effective than the 

 prairie dog and ground squirrel. In many areas the ridges produced by 

 his work as he makes his way underground are everywhere intersecting. 

 He works alike in field and in forest; in the South and in the North; in 

 the lowland and high up in the mountains under the snow. In the forests 

 of the mountains, just after the snow has melted, the intersecting ridges are 

 often so thick that a person can scarcely step without crushing one of them. 

 The rabbits in Australia: have multiplied in an amazing way. Their 

 burrows are innumerable. In Cape Colony there is said to be almost 

 constant movement of the ground due to the various animals. In various 

 parts of the West, especially in the semiarid regions, the ground has been 

 so penetrated with burrowing animals that in riding rapidly great care 

 must be taken lest one's horse break through the crust into the holes. 

 Blake says that in Tulare Valley, California, "mules often break through 

 the thin crust and sink to their shoulders in these holes." a 



Man. — Finally, man is the most important of all the mechanical organic 

 agents. He has stripped the soil of its protective vegetation, including 

 both grasses and forests, for large parts of the world. As a consequence of 

 this the transporting power of running water has been multiplied for these 

 areas many fold. Wherever there are fields the rivulets run turbid to the 

 brooks. Not only is the denuded soil transported seaward with man}- fold 

 the speed of natural conditions, but in many fields where cultivation is care- 

 lessly carried on or is neglected there form gulleys and deep ravines which 

 cut through the subsoil or to the very bottom of the belt of disintegration, 

 or even into the solid rock. A plexus of ravines once formed rapidly 

 deepens and extends, and thus carries on the process of transportation with 

 ever-accelerating speed. 



a Merrill, cit., p. 394. 



