412 GEOLOGY 



by passing into monoclinal folds (Fig. 38), and sometimes without 

 connection with folding. In depth they probably die out in similar 

 ways in most cases. A fault of thousands, or even hundreds of 

 feet is probably the sum of numerous slight slipping* distributed 

 through long intervals of time. The faulting along one plane may 

 be the cause of many earthquakes. 



The significance of normal and reversed faults. 1 Faults afford 

 an indication of the conditions of stress and tension to which a 

 region has been subjected, but some caution must be exercised in 

 their interpretation. Normal faults usually indicate an extension 

 of the surface sufficient to permit the fault-blocks to settle down 

 unequally. Reversed faults usually signify a compression of the 

 surface which required the blocks to overlap one another more 

 than before the faulting. In other words, normal faulting usually 

 implies tensions! stress, and reversed faulting compressional stress. 

 There may be cases of normal faulting in a region of compressed 

 and folded strata, and reversed faults in regions of tension; but 

 such cases are exceptional and usually local. These exceptional 

 cases aside, the general inference from prevailing normal faults is 

 that the regions where they occur have undergone stretching, while 

 the inference from the less widely distributed thrust faults is that 

 the surface where they occur has undergone compression. 



In view of the current opinion that the crust of the earth has 

 been subjected to great lateral thrust as a result of cooling, it is 

 well to make especial note of the fact that the faults which imply 

 stretching are called normal because they are the more abundant ; 

 and that the faults which imply thrust are less common, and are 

 styled reversed. The testimony of normal faults in favor of tension 

 is supported by the prevalence of gaping crevices, and of veins 

 which are but crevices that stood open until filled by deposition. 

 All these phenomena seem to testify to a stretched condition of the 

 larger part of the surface of the continents. 



Faulting may bring about numerous complications in the out- 

 crop of rock formations, as already illustrated (pp. 65-67). 



Faults of horizontal displacement. The preceding discussion 

 has concerned faults which involve more or less vertical displacc- 



1 Van Hise, Sixteenth Ann. Kept., U. S. Geol. Surv., Pt. I, pp. 672-678. 



