Field-Laborers 49 



more ways than one. In the first place, they have added 

 to the organic matter of the soil, and in the second, to the 

 mineral matter also; and besides this they have done much 

 to drain the soil, and expose it to the influences of the 

 sun and air. 



The organic matter which they have added, besides 

 their own droppings, consists of the materials which they 

 use to line their nests, principally leaves and grass, and 

 also the remnants of their food, nuts, grain, acorns, and 

 sometimes the whole of their winter stores. 



They have added also to the mineral matter of the soil 

 by helping on the decay of the underlying rocks. These 

 are seldom at any great depth beneath, for the loose 

 materials with which they are covered are but as a film of 

 dust compared with the thickness of the solid mass. The 

 soil at its very thickest is measured only by feet, while 

 the solid crust of the earth is measured at least by hun- 

 dreds of miles; and in most cases the soil is actually only 

 a few, often a very few, feet thick. 



In the western regions of North America, from Mexico 

 to the Arctic Ocean, as well as in the northern parts of 

 the Old World, there are a large number of small animals 

 called by the general name of ''ground-squirrels," and 

 resembling tree-squirrels in many respects, though some 

 of their habits are very different. Like the tree-squirrels, 

 they lay up stores of food, but unlike them, they burrow 

 in the ground, and live together in large villages instead 

 of in pairs. 



The gopher, or Canada pouched-rat, too, is to be 

 found in the prairie, where it dwells not merely in thou- 

 sands, but in hundreds of thousands, and has so completely 

 taken, perhaps we should rather say kept, possession that 



