These concavities of surface are so extensive and so widely distributed over the globe that no part of the outer shell can be supposed to be capable of accumulating notable stresses unless rigidly attached to the earth-body below. In other words, so far as sphericity is concerned, the crust must ease all its stresses nearly as fast as they accumulate, if, as usually assumed, it rests on a contracting or mobile substratum.
Surface cooling under these conditions should give only feeble thrusts, developed and eased nearly constantly. Such movements should be admirably adapted to give those gentle, nearly constant subsidences that furnish the nice adjustments of water-depth required for the accumulation of thick strata in shallow water, and those slow upward warpings that renew the feeding-grounds of erosion, the necessary complement of the deposition. These gentle, nearly constant movements mark every stage of geological history, and constitute one of its greatest though least obtrusive features. But if superficial stresses arising in this way are eased in producing these effects, they cannot accumulate to cause the great periodic movements.
Even where the crust is not concave, it is so warped and so traversed by folds and fault-planes that its resistance to thrust is relatively low, and it should, therefore, warp easily and at many points, if the thrust be confined to a superficial crust.
General conclusion.—When to the weakness of the crust, as computed under ideal conditions, there is added the weakness inherent in these concave and warped tracts, the conclusion seems imperative that while the crust is the pliant subject of minor and nearly constant warpings, such as are everywhere implied in the stratigraphic series, it is wholly incompetent to be the medium of those great deformations which occur at long intervals and mark off the great eras of geologic history. These great deformations apparently involve the whole, or a large part, of the body of the earth, and seem to require a very high state of effective rigidity.
General references on crustal movements.—Babbage, Jour. Geol. Soc., Vol. III (1834), p. 206 Lyell, Principles of Geology, Vol. II, p. 235; Mallet, Phil. Trans. (1873), p. 205; Reade, Origin of Mountain Ranges, and Evolution of Earth Structure; Fisher, Physics of the Earth’s Crust; Dutton, Greater Problems of Physical Geology, Bull. Phil. Soc. of Washington, Vol. XI, p. 52, also Amer. Jour. of Sci., Vol. VIII (1874), p. 121, and Geology of the High Plateaus of Utah (1880); Jamieson, Quar. Jour. Geol. Soc. (1882), and Geol. Mag. (1882), pp. 400 and 526; Heim, Mechanismus der Gebirgsbildung; Marjerie and Heim, Les Dislocations de l’Écorce terrestre (1888); Shaler, Proc. Boston Soc. Nat. Hist., Vol. XVII, p. 288; Dana, Manual of Geol., 4th ed., p. 345 et seq.; Woodward, Mathematical Theories of the Earth, Smithsonian Rept. for 1890, p. 196; Willis, The Mechanics of the Appalachian Structures, 13th Ann. Rept. U. S. Geol. Surv., Pt. II (1893), pp. 211–282; LeConte, Theories of Mountain Origin, Jour. Geol. Vol. I (1893), p. 542; Gilbert, Jour. Geol., Vol. III (1895), p. 333, and Bull. Phil. Soc. of Washington, Vol. XIII (1895), p. 31; Van Hise, Earth Movements, Trans. Wis. Acad. Sci., Arts and Let., Vol. II (1898), pp. 512–514; Estimates and Causes of Crustal Shortening, Jour. Geol., Vol. VI (1898), pp. 29–31; Relations of Rock Flowage to Mountain Making, Mon. XLVII, U. S. Geol. Surv. (1904), pp. 924–931; A. Geikie, Text-book of Geology, 4th ed., pp. 672–702.
CHAPTER X.
THE EXTRUSIVE PROCESSES.
Outward movements.—In the preceding chapters movements toward the center have been considered. The complementary processes of outward movement now invite attention. Without doubt these are mainly but a resultant of the centripetal actions. For each pound of material moved outwards an equivalent is quite surely moved inwards. Notwithstanding this, the outward movements have a peculiar nature of their own, and serve a function of radical importance in the economy of the globe. Some minor phases have been incidentally considered, such as the upward flow of springs and deep-seated waters, but here the descending and ascending factors are alike, and are closely and obviously connected.
VULCANISM.
The great example of ascensive action is the movement of fluid rock from the interior outwards. The term vulcanism will be used to embrace not only volcanic phenomena in the narrower sense, but all outward forcing of molten material, whether strictly extrusive or merely ascensive.