ORIGIN OF FRACTURES AND FAULTS. 727 



2. Faults. 



The several causes which may produce fractures through layers 

 or strata may also cause faults, and the profounder the fractures 

 the more extensive the faults that may result. 



In general, there is a dropping down of the rock on one side of 

 the fracture by gravity ; and, where the fault is a sloping one (as 

 in fig. 978), the mass on the upper side usually slides down the 

 sloping plane. But, under the action of the pressure which has 

 plicated the earth's crust and lifted mountains, the reverse move- 

 ment has not been uncommon. The figure referred to is an exam- 

 ple ; and, by the upthrow, rocks of the Lower Silurian have been 

 carried up to the level of those of the Subcarboniferous. Similar 

 faults may occur along the axes of plications as described by the 

 Professors Eogers. 



In faulting, there may be either a vertical or lateral slide, or an 

 oblique one. The inequality of the faulted parts of the veins repre- 

 sented in the figures on page 121 is accounted for on the ground 

 of a lateral or oblique slide. 



The strata sometimes have a different dip on the sides of a fault (figs. 96, 

 97). This may arise in different ways. 



1. The plane of fracture may not have the same slope in its different parts, 

 so that in either a vertical or lateral slide parts of unequal dip are Drought 

 together. 



2. The rocks may open at the fault, and the parts be adjusted together by 

 wear of the sides during the downthrow or uplift; and any portion of the 

 fissure remaining open may be filled by the rubbish thus produced. 



3. Wedge-shaped plates, larger below, may separate and fall, leaving the 

 rock either side of the vacated space to be pressed together by the breaking- 

 force. 



4. Fractures converging downward may separate wedge-shaped masses; 

 and the rock on one side or the other may fall off some degrees, while the wedge 

 settles into its new position by gravity, and is adjusted to it by friction in the 

 descent. If the rock to be faulted has a considerable dip transverse to the direc- 

 tion of the force, lateral slides would be of very common occurrence. 



5. Plications and uplifts may take place, after a profound fracture in the 

 rocks, on one side of the fracture, and not on the other. The abrupt transition 

 in many places between the plicated region of the Appalachians and the 

 slightly-tilted rocks of the country northwest of them can have no other ex- 

 planation. 



3. Structural peculiarities: slaty cleavage and jointed 

 structure. 



1. Slaty cleavage. — Slaty structure (exemplified in figs. 89 to 91, p. 

 101) has been shown, by Sharpe, Sorby, Darwin, and others, to be 



