. Elementary physical geography . bysome of these rocks. Attimes there are unconform-ities (Figs. 102 and 103):that is, rocks made in thesea, rest on other sea-formed strata which were deposited at an earlier period,and have since been land. Thus we have in these cases,(1) deposit in the ocean, (2) elevation to land, (3) depressionbeneath the sea, and (4) a second elevation. In some casesthere are numerous such unconformities, showing successivechanges. These, and other facts, prove that the crust of the earth is almost con-stantly in ofSedimentary Rocks.— The rocks of t
. Elementary physical geography . bysome of these rocks. Attimes there are unconform-ities (Figs. 102 and 103):that is, rocks made in thesea, rest on other sea-formed strata which were deposited at an earlier period,and have since been land. Thus we have in these cases,(1) deposit in the ocean, (2) elevation to land, (3) depressionbeneath the sea, and (4) a second elevation. In some casesthere are numerous such unconformities, showing successivechanges. These, and other facts, prove that the crust of the earth is almost con-stantly in ofSedimentary Rocks.— The rocks of thesea are soft and un-consolidated, whilethose of the land aregenerally hard andcompact. The consolidation of rocks is a simple process,generally resulting from heat, pressure, the deposition ofsome cement, or a combination of several such causes. In ahydraulic press we can consolidate clay; and in a similarway, the great weight of the strata of the crust, furnishesthe necessary pressure for the natural consolidation of Fig. 103. An unconformity in inclined rocks,land surface. A, B, old 218 PHYSICAL GEOGRAPHY. Bricks are consolidated by heat, and in the earth heat oftenacts in a similar manner. All rocks in the earth are filledwith water which is slowly percolating through them. Thiswater is dissolving substances from one place and depositingthem in others, and in this way many rocks are being con-solidated. Carbonate of lime and some compound of iron,are the common rock cements ; and these, perhaps aided byone of the other causes, bind the rock particles together. Geological Chronology. — By a study of the rocks, the mainfacts of geological history have been determined in a moreor less satisfactory manner. We know something of thehistory of the globe, and the rocks form the pages and chap-ters of this history. The rock record is often very imper-fect. Some pages, and at times entire chapters, are missing ;but enough still remains to furnish a basis of value. Onethi
Size: 2663px × 938px
Photo credit: © Reading Room 2020 / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookauthortarrralphsralphstockm, bookcentury1900, bookdecade1900
