CHAPTER III. 



FAULTS. 



IT lias been pointed out that some faults are due to 

 the unequal shrinkage of .strata ; others to movements 

 in the earth's crust. The latter, however, are, for the 

 most part, indirectly connected with shrinkage, as it 

 is the contraction of the interior of the earth that 

 causes the movements in the crust. Some faults are 

 due to the subtraction of matter, others to its expan- 

 sion ; as matter may be subtracted either by running 

 water or volcanic action, and expansion may be due 

 to vulcanism or chemical action. 



As the composition of the rocks forming the 

 earth's crust varies, not only in passing from one to 

 another, but also in different parts of the same 

 mass (as one part of a bed may be more argillous, 

 arenaceous, or calcareous than another), different beds, 

 and even different parts of the same bed, may con- 

 tract unevenly. The Caudelaria lode, in the district 

 of Chanareillo, Chili, " is seldom more than six 

 inches wide when it crosses the hornblendic dykes ; 

 but though uniting with no other veins, it enlarges 



