. Elements of geology : a text-book for colleges and for the general reader. Geology. 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
. Elements of geology : a text-book for colleges and for the general reader. Geology. 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-. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original LeConte, Joseph, 1823-1901. New York : D. Appleton and Co.
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