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and of the normal crust to be the same.  The sediments are, however, less radioactive in the proportion of 4 to 3.  Nevertheless the effects of the increased thickness will be considerable.

Now this remarkable increase in the temperature arises entirely from the condition attending the radioactive heating; and involves something additional to the temperature conditions determined by the mere depression and thickening of the crust as in the Babbage-Herschel theory.  The latter theory only involves a shifting of the temperature levels (or geotherms) into the deposited materials.  The radioactive theory involves an actual rise in the temperature at any distance from the surface; so that the level in the crust at which the rocks are softened is nearer to the surface in the geosynclines than it is elsewhere in the normal crust (Pl.  XV, p. 118).

In this manner the rigid part of the crust is reduced in thickness where the great sedimentary deposits have collected.  A ten-kilometre layer of sediment might result in reducing the effective thickness of the crust by 30 per cent.; a fourteen-kilometre layer might reduce it by nearly 50 per cent.  Even a four-kilometre deposit might reduce the effective resistance of the crust to compressive forces, by 10 per cent.

Such results are, of course, approximate only.  They show that as the sediments grow in depth there is a rising of the geotherm of plasticity—­whatever its true temperature may be—­gradually reducing the thickness of that part

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of the upper crust which is bearing the simultaneously increasing compressive stresses.  Below this geotherm long-continued stress resolves itself into hydrostatic pressure; above it (there is, of course, no sharp line of demarcation) the crust accumulates elastic energy.  The final yielding and flexure occur when the resistant cross-section has been sufficiently diminished.  It is probable that there is also some outward hydrostaitic thrust over the area of rising temperature, which assists in determining the upward throw of the folds.

When yielding has begun in any geosyncline, and the materials are faulted and overthrust, there results a considerably increased thickness.  As an instance, consider the piling up of sediments over the existing materials of the Alps, which resulted from the compressive force acting from south to north in the progress of Alpine upheaval.  Schmidt of Basel has estimated that from 15 to 20 kilometres of rock covered the materials of the Simplon as now exposed, at the time when the orogenic forces were actively at work folding and shearing the beds, and injecting into their folds the plastic gneisses coming from beneath.[1] The lateral compression of the area of deposition of the Laramide, already referred to, resulted in a great thickening of the deposits.  Many other cases might be cited; the effect is always in some degree necessarily produced.