246 DISLOCATIONS OF STRATA 



faulted strata are horizontal, the vertical throw is the same as the 

 stratigraphic, but if the strata are inclined, the two differ. 



Though the term fault is applied to any dislocation of strata, 

 by which the broken ends of the beds are carried over or past 

 each other, yet it includes structures of very different significance, 

 produced in dissimilar ways. Of these there are two principal 

 classes : — 



I. Normal, or Gravity, Faults (Figs. 101, 102) are those in 

 which the hade is toward the downthrow, or depressed side, and 

 thus the hanging wall is on the latter side, while the foot wall 

 forms the upthrow side. The phrase "normal fault" is an unfortu- 

 nate one, but it seems to be too deeply fixed in geological usage 

 for any change to be practicable. Faults of this class ordinarily 

 occur in horizontal or moderately inclined or folded rocks. As 

 will be seen in a later section, normal faults in horizontal or gently 

 inclined strata imply the extension of an arc of the earth's crust, 

 because the faulted beds occupy more space, horizontally, than 

 they did before dislocation. They are therefore due to a tension 

 greater than the rocks could endure. 



The normal faults of any district may nearly always be classed 

 in two categories : ( 1 ) Strike faults, which, in general, follow 

 jtuj^JijtXjwbft strike of the beds. To this group belong the great faults, — 

 great both as to length and amount of displacement. (a) Dip 

 faults. These are more or less parallel" to the dip of the" strata, 

 and therefore form an open angle with the strike faults of the 

 same area : they are less important than the latter. 



The amount of displacement of faults varies greatly in different 

 cases, from a fraction of an inch up to thousands of feet. In 

 those of small throw the fault plane is frequently a clean, sharp 

 break ; but in the greater faults the rocks in the neighbourhood 

 of the fault plane are often bent, crushed, and broken. The fault 

 itself is then filled up with a confused mass of fragments of all 

 sizes, which may be cemented into a breccia. Such a mass is 

 called fault rock. In soft rocks the fault plane is always closed 

 by the immense weights and pressures involved, but in rigid 

 rocks it may remain more or less open, especially if the break be 



