384 VETEEINAEY HYGIENE 



there is no perceptible increase in the amount as compared 

 with countries which are comparatively new, and this is 

 due to the fact that the soil is capable of regeneration. 



In cities the chances of regeneration are more remote, so 

 that in the soil of towns and cities more organic matter is 

 met with as the result of human agency than in country 

 places. Plants by their death and decay contribute to 

 earth pollution, but here again, excepting the material be 

 so excessive that the local conditions cannot cope with it, 

 the remains of the plants are gradually by the earth's 

 process of self-purification removed. 



A study of soils is of greater value to the agriculturist 

 than to the physician, there is no mistaking the part they 

 play in the growth of the plant, and from a veterinary point 

 of view soils are of much more interest to us agriculturally 

 than pathologically. The one aspect is definite and inti- 

 mately connected with the welfare of animals, the other is 

 to some extent doubtful, though there are of course con- 

 ditions of soil which are undeniably connected with health. 



Soils, Geologically. — The interior of the earth is composed 

 of rocks which may broadly be divided into two great 

 groups, those arranged in layers known as stratified, and 

 those which present no stratification. Stratified rocks from 

 their having been built up in successive layers under water 

 are sometimes known as aqueous, while the non- stratified, 

 which have resulted from the cooling down of what was 

 originally molten material, are sometimes known as 

 igneous. A familiar example of the latter is granite, and 

 of the former sandstone. 



The study of geology reveals that the stratified group of 

 rocks are capable of being classified according to their age 

 and distinctive characters ; we cannot attempt here to deal 

 with this question even in outline, our main point is com- 

 prised in the statement that no matter what the nature of 

 the rock may be, the crust of the earth or soil is entirely 

 derived from the rock beneath, so that soils are regarded 

 as rocks which have worn down or otherwise become dis- 

 integrated, or as the geologist terms it, ' weathered.' 



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