ROTATORY MOVEMENTS 179 



Overthrusts: These are reverse faults with low dip or large hade. In 

 some cases the dip-slip has been enormous, amounting to tens of kilo- 

 meters. Scott calls them "thrusts" and separates them entirely from 

 faults of high dip; but the word "thrust" has been used for ordinary 

 reverse faults of high dip. The word "overthrust" has been very gener- 

 ally used for this kind of fault and is very descriptive. It should be 

 adopted. 



Flaws: Suess has described with care certain faults in which the strike 

 is transverse to the strike of the rocks, the dip high and varying from one 

 side to the other in the course of the fault, and the relative movement 

 practically horizontal and parallel with the strike of the fault. He has 

 used the word "blatt" (plural, "blatter") to designate them. Miss Sollas 

 has used the word "flaw" in the English translation of Suess. 



Eotatory Movements 



As has already been pointed out, the displacements of all large faults 

 will be accompanied by a rotation varying in amount along the course of 

 the fault; moreover, in some cases the general character of the displace- 

 ment may be rotatory, although the rotation will not be uniform along 

 the fault. Where rotations occur, the amount of rotation of the part 

 under consideration and the direction of the axis should be given. Eota- 

 tory movements on faults have been but little studied, and it is not con- 

 sidered advisable to suggest a special nomenclature at present. Faults 

 with rotatory movements have been called "pivotal" faults ; but this term 

 is not satisfactory, as there are no pivots. 



Groups of Faults 



It seems unnecessary to define all combinations of faults. The words 

 in common use are sufficiently consistent and universal not to cause con- 

 fusion. There is one group, however, that might advantageously be de- 

 fined. Where a mass is elevated or depressed, it is sometimes surrounded 

 by one set of faults, cut approximately at right angles by a second set. 



Peripheral faults are faults running along the periphery of a geolog- 

 ically elevated or depressed region. 



Radial faults are faults radiating roughly from a point. Each of these 

 two groups of faults may exist without the presence of the other. Scott 

 has divided all faults into "thrust," "radial," "horizontal," and "pivotal" 

 faults; the first have been called "overthrusts" above, and the next two 

 are roughly our dip-slip and strike-slip faults of high dip. By the word 

 "radial" Scott apparently refers to the directions of the earth's radii. 

 XIII— Bull. Gbol. Soc. Am., Vol. 24, 1912 



