PART I. 



SOILS. 



CHAPTER I. 



GEOLOGICAL, STRATA. 



TF we make borings into the earth or study railway 

 cuttings by hill sides, we rind the earth and rocks exposed 

 are of different character and consistency, and we notice, as 

 a rule, well-marked stratifications both in the loose earth 

 and the hard rocks so exposed. The deepest mine is only 

 about \ a mile in depth, and so we can study the soils and 

 rocks only of the outer crust of the earth. As the rocks, 

 however, do not always occur in horizontal layers, and as 

 the crust of the earth has evidently undergone violent con- 

 tortions, we are able actually to study rocks situated down 

 to a depth of about 20 miles, on the very surface of the 

 earth. In other words, there are rocks on the surface of 

 the earth which would have been buried 20 miles deep had 

 not violent eruptions brought them up to the surface. The 

 evidences for such eruptions are numerous. We find the 

 temperature of the crust of the earth increases by about iF. 

 for every 56ft. of depth. The deeper we go down in a mine 

 the warmer it is, and we can only imagine how hot it is 

 20 miles below the surface of the earth. It is over 2000 F. 

 a temperature at which all minerals and rocks must be in a 

 fluid and disturbed condition. We have further evidence 

 of this internal heat in hot-springs, earthquakes and vol- 



