Physics of the Earth - II The Figure of the Earth Bulletin of the National Research Council (1931) / Chapter Skim
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Chapter XIII. On Some of the Greater Problems of Physical Geology
Chapter XIII. On Some of the Greater Problems of Physical Geology
Pages 201-211
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From page 201... ...
The objection to this explanation is twofold: Ifs the first place, we cannot, without resorting to violent assumptions, find in this process a sufficient amount of either linear or volume contraction to account for the effects attributed to it. :[n the second place, the distortions of the strata are not of the kind which could be produced by such a process.
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From page 202... ...
Here is a great belt of parallel synclinals and a,nticlinals with a persistent trencl, and no rational inquirer can doubt that, they have been puckered up by some vast force acting horizontally in a northwest and southeast direction. Doubtless it is the most wonderful example of systematic plication in the world.
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From page 203... ...
in that wonderful belt, though tens of thousands of feet in thickness, were all deposited in comparatively shallow water. The Paleozoic beds of the Appalachian region have a thickness ranting from 10,000 to over 30,000 feet, yet they abound in proofs that when they revere deposited their surfaces were the bottom o:l a shallow sea.
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From page 204... ...
The flanks of these platforms, with the upturned edges of the strata reposing against them or with gigantic faults measuring their immense uplifts, plainly declare to us. that they have been slowly pushed upwards as fast as they were degraded by secular erosion.
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From page 205... ...
Thus the geologic changes which have taken place may be regarded as experiments conducted by Sat.ure herself on a Vast scale, and from her experiments w ~ may by suitable working hypotheses draw provisional conclusions, both as to the degree in which the earth approximates to isost.asy and also as to the mean effective rigidity of large portions of the subterranean mass. The approach to isostasy is thereby inferred to be very near, while the mean rigidity of the subterranean masses is.
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From page 206... ...
the indications are more systematic. On both the Atlantic and Pacific shores the deflection of the plummet is almost invariably towards the ocean, and is often of considerable amount; but it is along the shore that the isostatic theory would lead us to look for just this deflection, for it is along the margins of the continents that great bodies of sediment accumulate; and so long as the earth possesses any noteworthy degree of rigidity, enabling it to sustain in part the resulting (reformation of isostas.y, so long must we expect to kind these sediments constituting- an excess of masts whose attraction will make itself felt upon the plummet.
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From page 207... ...
Taking, then, the case of a land area undergoing denudation, its detritus carried to the sea and deposited in a heavy littoral belt, we may re~a.rd the weight of each elementa.ry part of the deposited mass as a statical force acting upon a viscous support below. Assuming that we could find a deferential expression appliea.ble to each and every element of the mass and a.
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From page 208... ...
we assume for the mean Viscosity of the superficial and subterranean masses involved in the movement a much greater value than I am disposed to concede. The result is ~ true viscous flow of the loa.decl littoral inward upon the unloaded continent.
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From page 209... ...
and places required, and one which has the intensity and amount required and no more. The contractional theory gives us a force having neither direction nor determinate mode of action, nor definite epoch of action.
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From page 210... ...
Geologic history discloses the fact that some great areas of the earth's surface which were in former ages below sea level are now thousands of feet above it. It also gives us reason to believe that other areas now submerged were in other ages terra 1?
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From page 211... ...
If the increase of' volume of an elevated area be due to an accession of matter, the plateau must be hoisted against its own rigidity and also against the statical weight of its entire mass lying above the isostat,ic level. But if' the increase of' volume be due to a decrease of density there is no resistance to be overcome in order to raise the surface.
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