FISSURES, OR FRACTURES. 



233 



The explanation of normal faults is not so obvious. In the case 

 of great faults of this kind the explanation is probably as follows: 

 Suppose a portion of crust lifted by intumescence of sub-crust layer, 

 produced either by access of water from above or by hydrostatic press- 

 ure transferred from a subsiding area in some other perhaps distant 

 place. The crust would be broken by more or less parallel fissures 

 into great oblong blocks many miles in extent. Since the fissures 

 are usually more or less inclined, these crust-blocks would be either 

 rhomboidal or wedge-shaped (Fig. 206, A). As the crust rose into 

 an arch these blocks would separate (Fig. 206, B). As soon as the 



A- 



* /* \JL 



L 



y_ 



I 



Fig. 206.— Diagrams showing how Normal Faults are probably formed. 



tension is relieved by escape of elastic vapors or lava or both, the blocks 

 would readjust themselves by gravity into new positions. In doing 

 so the rhomboidal blocks ab f ' g would tilt over on the overhanging 

 side and heave up on the obtuse-angle side, producing in every case 

 normal faults, and the wedge-shaped blocks c d e would sink bodily 

 lower or float bodily higher according as the base of the wedge was 

 upward or downward, producing again in every case normal faults, as 

 shown in Fig. 206, G. The result of such readjustment of crust-blocks 

 is admirably shown on a large scale in the structure of the Basin re- 

 gion, and especially in Southeastern Oregon (Fig. 207). The fractur- 



Fig. 207.— Sketch section through Warner and Abert Lakes, S. E. Oregon (after Russell). W. L., 

 Warner Lake; A. L., Abert Lake ; C/i. Y., Chemaukan Valley. 



ing and tilting here have been so recent (beginning of Quaternary) 

 that erosion has had little effect in modifying the orographic forms. 

 It is seen that the upheaved side of every crust-block forms a mount- 

 ain ridge, while the dropped side forms a valley on which drainage 

 waters accumulate to form a lake. 



