. Elements of geology : a text-book for colleges and for the general reader. Geology. 180 STRATIFIED OR SEDIMENTARY ROCKS. strata-edges exposed, to sink down and become ocean-bottom, and re- ceive sediments covering the strata-edges and filling the erosion-hol- lows, and afterward to rise again and be submitted to the inspection of the geologist; Fig. 157 represents in section what he would see. They are interpreted as A follows : In A and B the lower series of strata was first deposit- ed ; then the sea- bottom was raised to land - surface and the strata tilted and eroded; then it went down a
. Elements of geology : a text-book for colleges and for the general reader. Geology. 180 STRATIFIED OR SEDIMENTARY ROCKS. strata-edges exposed, to sink down and become ocean-bottom, and re- ceive sediments covering the strata-edges and filling the erosion-hol- lows, and afterward to rise again and be submitted to the inspection of the geologist; Fig. 157 represents in section what he would see. They are interpreted as A follows : In A and B the lower series of strata was first deposit- ed ; then the sea- bottom was raised to land - surface and the strata tilted and eroded; then it went down again and re- ceived the upper series; and, final- ly, was raised and inspected by the geologist. In C the process was the same, except that the first se- ries of strata was raised without tilting. In D the second series of strata ivas also tilted in the second raising. Now the strata of each series are said to be conformable among themselves, but the two series are unconformable with one another. Definition.—Therefore, strata are said to be conformable when they are parallel, continuous, and therefore formed under the same condi- tions, and are unconformable when they are discontinuous, and formed under different conditions ; the discontinuity being ahvays marked by an old eroded land-surface. Unconformable strata are usually non- parallel, and this is often made a part of the definition; but this is not necessary. In Fig. 157, C, there is no want of parallelism. The reason we have already explained. A section like any one of the foregoing—among the commonest in geology—reveals many interesting events : 1. A long period of quiet, during which sediments of the first series were deposited continuously on a sea-bottom. The length of this period is measured by the thick- ness of the series. 2. A period of elevation, during which the sea- bottom became land-surface. We have no means of estimating the length of this period. 3. A long period during which the land-surface became
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectgeology, bookyear1892
