GENERAL TERMS 165 



may have been learned about a fault in simple, definite language before 

 the whole movement and the force which has caused it are known; in 

 many cases these will never be known. A dynamic system of classification 

 and nomenclature will be in order when the dynamics of faults is better 

 understood. 



Spurr 3 and Tolman 4 have proposed systems of nomenclature for fault 

 movements which are feasible ; the latter's system is especially logical and 

 full. The objections to both are that they break with the best usage, and 

 that the terms do not suggest the component of the displacement denoted 

 by them. These are serious objections. A strong effort has been made to 

 render the system here proposed not open to these objections. 



De Margerie and Heim 5 have collected the terms used to describe faults 

 up to 1888. But little is said of strike-slip faults and they do not suggest 

 a consistent nomenclature, but merely collate the terms used. 



Geological text-books give but few terms descriptive of fault move- 

 ments. Those of Scott, 6 A. Geikie, 7 and J. Geikie 8 are the fullest. 



Our present nomenclature has developed largely in relation to move- 

 ments up or down the fault plane. It becomes inadequate when move- 

 ments occur parallel with the fault strike, movements now recognized as 

 very general. It is important, therefore, to extend the meaning of the old 

 terms and to introduce some new ones, but in a way not to disturb the 

 older usage nor to introduce any confusion by the new applications. 



In what follows many definitions are given for the sake of continuity 

 which might otherwise be omitted. 



General Terms 



A fault, in its simplest form, is a fracture in the rock of the earth's 

 crust, accompanied by a displacement of one side with respect to the 

 other in a direction parallel with the fracture. The fracture is usually 

 not an open crack, and an open crack would not be a fault unless one of 

 the sides had moved parallel with the crack relatively to the other. A 

 monoclinal displacement, however steep, is not a fan 11 ; but it might, 

 by tearing, pass into a fault. Faults rarely consist of a single clean-cut 

 fracture; they are usually made up of a number of fractures, as noted 

 below. 



8 Geology applied to mining. New York, 1904. 



♦Economic Geology, vol. II, pp. 506-511. Graphical solution of fnult problems. San 

 Francisco and London, 1911. 



8 Les Dislocations de l'ecorce terrestre. Zurich, 1888. 



6 An Introduction to Geology, 2d ed., 1907. 



7 Text-book of Geology, 4th ed., 1903. 

 • Structural and Field Geology, 1905. 



