. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. Slope of valley 20°, dip of strata 20°, in opposite directions. Fte- 76- a valley, which declines in an opposite direction at 20°.* These rules may often be of great practical utility; for the different degrees of dip occurring in the two cases represented in figures 74 and 75, may occasionally be en- countered in following the same line of flexure at points a few miles distant from each other. A miner un- acquainted with the rule, who had first explored the valley (
. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. Slope of valley 20°, dip of strata 20°, in opposite directions. Fte- 76- a valley, which declines in an opposite direction at 20°.* These rules may often be of great practical utility; for the different degrees of dip occurring in the two cases represented in figures 74 and 75, may occasionally be en- countered in following the same line of flexure at points a few miles distant from each other. A miner un- acquainted with the rule, who had first explored the valley (fig. 74), may have sunk a vertical shaft below the coal-seam A, until he reached the inferior bed B. He might then pass to the valley fig. 75, and discovering there also the out- crop of two coal-seams, might begin his workings in the uppermost in the expectation of coming down to the other bed A, which would be observed cropping out lower down the valley. But a glance at the section will demonstrate the futility of such hopes. In the majority of cases, an anticlinal axis forms a ridge, and a syn- clinal axis a valley, as in A, B, fig. 62, p. 48; but there are exceptions to this rule, the beds sometimes sloping in- wards from either side of a mountain, as in On following one of the anticlinal ridsres Fig. of the Jura, before mentioned, A, B, C, fig. 71, we often discover longitudinal cracks and sometimes large fissures along the line where the flexure was greatest. Some of these, as above stated, have been en- larged by denudation into valleys of considerable width, as at C, fig. 71, which follow the line of strike, and which we may suppose to have been hollowed out at the time when these rocks were still beneath the level of the sea, or perhaps at the period of their gradual emergence from be- neath the waters. The existence of such cracks at the point of the sharpest bending of solid strata of limestone is precisely what we should have expected; but the occasional wan
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Keywords: ., bookcentury1800, bookdecade1860, booksubjectgeology, bookyear1868
