PART II

ORIGIN OF REGIONAL VERTICAL MOVEMENTS OF THE

EARTH'S CRUST

While evidences of these regional movements have accumulated with the progress of geology, the attempts to explain their origin can hardly be said. to have advanced at the same rate.

Incompetence of the Principle of Isostacy to explain Areal Oscillations of Level. The principle of isostacy has been appealed to, but the mass of the solid earth involved in these movements is so vast, compared with any sedimentation that has taken place in the same period of time, that such an explanation is quite futile.

One cubic mile of sediment pressing down the earth's crust, say, 1,000 feet could not lift two cubic miles of another portion of the earth's crust 1,000 feet high, whatever mobility the undercrust may be assumed to possess. Yet I venture to affirm, if we can rely upon the observations quoted, that the movements of the earth's crust in mass in Pleistocene times have exceeded by twenty times the mass of sediment contemporaneously denuded from the land and laid down in the sea.

Many thinkers, from an early period in the study of geology, seeing that while one portion of the earth's surface has been elevated another has been depressed, have considered the phenomena to be related as cause and effect, looking upon these

opposite movements as contemporaneous; but why they should stand in this relation is not apparent. The explanation present to their minds appears to have been that in some unknown way there was a transference of material from areas of subsidence to areas of elevation, which, of course, must have taken place either in or under the earth's crust. An adequate cause of such a transference on the scale required is difficult to conceive.1

The additional mass of material added or pushed up in one area would have to be balanced by an additional weight added to the depressed area. A shifting of weight by denudation and sedimentation we can conceive; but, as we have seen, it is insufficient, and such transference, even if it bent down the crust, would not cause hollows or apparent depression, but rather filling-up. Where, then, can the extra weight in the depressed area be derived from to balance the extra mass in the upheaved area, for if one movement were consequent upon the other some such transference of material would seem to be required? We can scarcely appeal to secular cooling of the earth as an efficient cause, nor am I aware that any geologists have done so. It may, however, be thought that the cooling and falling in, or bending, of the crust can in some way produce this effect; but I

Lyell in his Principles of Geology devotes chap. xxxiii. vol. ii. (tenth edition) to a discussion of many of these questions. Science has advanced since this was written, but it is well worth reading.

have a difficulty in following out such a conception. Secular refrigeration, it appears to me, would act cumulatively in one direction, whereas the movements of the earth's envelope are essentially slow pulsations.

Not due to External or Internal Transferences of Material from one Locus to another.-On a full consideration it seems highly improbable that these movements are due in any great measure to either an external or internal transference of material from one area to another. The balance of pressure must be preserved within certain limits, and I find it impossible to think of any force or agency at work in the earth's interior tending to produce such a movement; but even if there were such an agency, its effect would be limited by the possible deformation the earth could stand and retain. I venture to think that if the specific gravity of the materials of the earth were identical in each of the zones from the surface to the centre, even though the earth were as rigid as steel, the present configuration or inequalities of the levels. of the earth's surface could not be retained.1

1

Dr. G. H. Darwin, in a paper · On the Stresses due to the Weight of Continents' (Phil. Trans. of the Royal Society, 1882), estimates that the stress-difference under the continents of Africa and America is at a maximum at more than 1,100 miles from the earth's surface, and there amounts to about 4 tons per square inch, and he remarks that marble would break under this stress, but that strong granite would stand (p. 229). In this calculation he appears to have halved the heights to allow for the smaller density of the surface rock.

Dr. Darwin in this paper investigates several problems of this nature, working from certain hypothetical assumptions, and the results are most interesting. A consideration of the whole subject leads me

Mean Specific Gravity of the Elevated Portions of the Crust of the Earth less than the Mean Specific Gravity of the Whole Crust.-If these conceptions have an element of truth in them, the mean specific gravity of the elevated parts of the earth, and the foundations on which they rest-— that is, the continents and their mountains, and the under-mass of the earth-must be less than the specific gravity of the earth's crust and interior mass underlying the deepest depressions. So far as pendulum observations inform us, the fact appears to be established that the earth has a higher specific gravity under the oceans than it has under the continents.1

These observations, limited though they be, tend to show that though the levels of the earth's

to think that such stress-differences could not be maintained through geological time. We have seen that the earth is mobile and the changes it undergoes multitudinous, if slow. Readjustments are continually taking place. It is also open to doubt whether with such stress-differences a state of equilibrium sufficiently stable for the preservation of the existing inequalities of the earth's surface could exist even were the earth an inert body, which it certainly is not.

1 See Results of a Transcontinental Series of Gravity Measurements,' by G. R. Putnam, with notes by G. K. Gilbert, Phil. Soc. of Washington Bul., vol. xiii. pp. 31-76. See also Chap. xv., Physics of the Earth's Crust, second edition, where Mr. Osmond Fisher discusses the 'Revelations of the Pendulum.' Major S. G. Burrard, after a lengthened series of observations on the attractions of the Himalaya Mountains on the plumb-line, finds certain discrepancies which drive him to the conclusion that the undiscovered cause of the disturbance is traceable to a great invisible chain, of excessive density, traversing India from Balasore, near the mouth of the Hooghly, to Jodhpur in Rajputana, and underlying Mandla and Bhopal, or, roughly speaking, running parallel with the Himalayan chain (Nature, May 1902, pp. 80-82). That there exist considerable local variations of density in the earth's crust is extremely probable.

surface are variable, there exists in the earth's interior an equality of stresses, and that the protuberances do not create stresses on their foundations tending to force them down and, by displacing the under-layers, bring about an equality of surface levels. If the protuberances-by which I mean those portions of the continents and islands that are above the mean spheroidal level-represent so much additional material piled upon a statically balanced spheroid, it seems to me that a gradual deformation. must be taking place and a sinking of the higher lands. Should this be the case, what force exists within the earth to prevent the continued effects of this weighting and the natural removal of prominences above the sea-level to one uniform plane? If no such force exists, or has existed, the earth would long have ceased to possess the diversified features of land and water, so favourable to the habitation of man, and which have taken him so long to discover and map out.1

1 Herschel, with great acumen, has pointed out that if we would divide the globe into two hemispheres, the one of which shall contain the greatest quantity of land, and the other of water, it must be cut by a plane perpendicular, not to the axis of rotation, but (singularly enough) to the diameter, passing through the south-west corner of England. The fact is instructive, as it proves the force by which the continents are sustained is one of tumefaction, inasmuch as it indicates a situation of the centre of gravity of the total mass of the earth somewhat eccentric relatively to that of the general figure of the external surface the eccentricity lying in the direction of our antipodes-and is therefore a proof of the comparative lightness of the materials of the terrestrial hemisphere' (Physical Geography, pp. 14-15, fifth edition). Referring to the same phenomenon, Dr. Darwin in the paper already quoted says (p. 230) it has been impossible for him in that paper to take any notice of the stresses produced