872 STUDIES FOR STUDENTS 



very simplicity of areal distribution allow of several hypotheses, 

 any one of which would adequately explain them. 



NATURE OF EVIDENCE FOR ESTABLISHING A FAULT. 



The several ways in which individual faults may be discovered 

 the writer has elsewhere recited. 1 Some of them are .alone 

 sufficient as proof, while others must be regarded as offering 

 incomplete evidence to be weighed in connection with additional 

 observations. As modified for the problem before us these 

 methods will now be considered. Many of them could apply 

 equally to thrusting or to normal faulting, but observation of 

 fault hade will generally be sufficient to determine which in a 

 given case is present. 



1 . The observation of beds formed at different times in juxtaposi- 

 tion along a plane transverse to their beddi7ig. — With conformable 

 formations such an observation may be regarded as absolute 

 proof of a fault, and is, in fact, one concerning which there is no 

 disagreement among geologists. It is only rarely, however, that 

 such a contact will be found exposed, and hence the method 

 serves but seldom. 



2. Offsettiiig of formations in outcrop. — Almost equally reliable 

 as a method for determining a fault is the offsetting of forma- 

 tions in outcrop. If in following the mutual boundaries of two 

 formations, the formations are found abruptly thrown to the right 

 or left and continued either along the old or in a new direction, 

 it is only necessary to show that the core of a pitching fold has 

 not conditioned the break of continuity to prove the presence 

 of a cross fault. When several such faults occur in series with 

 throw in the same sense, either the apparent course of the 

 boundary as a whole is given a different rectilinear direction, or 

 a crescentic outline is conditioned. The crescentic ridges which 

 are so prominent a feature of the Connecticut valley are pro- 

 duced in this way. The three ways in which such ridges may be 

 produced by normal faulting have been elsewhere pointed out. 2 



3. Offsetting of outcrops as definite topographic features. — With 

 rocks which are resistant to the erosive agencies, or where fault- 



1 Twenty-first Annual Report, U. S. Geo/. Survey, Part III, pp. 85-93. 



2 Hobbs, loc. cit., pp. 95-8. 



