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were made to despatch a reconnaissance expedition during the summer of 1921 so as to determine the best direction for attempt- ing the ascent, which should be undertaken in 1922. Col. C. Howard Bury was appointed chief of the expedition, and Mr. H. Raeburn as leader of the reconnaissance. (O. J. R. H.)

GEOLOGY. Progress in scientific geology during 1910-21 is here discussed in four sections: (i) cosmic, (2) dynamical, (3) structural and (4) stratigraphical.

I. COSMIC GEOLOGY

Cosmic geology (see 11.648) deals with three main groups of problems: (i) the early history of the earth as inferred from its composition and structure, and from analogy to other heavenly bodies; (2) the physical conditions which by their influence on climate have controlled the origin and evolution of life on the earth; (3) the plan of the earth or distribution of continents and oceans as determined by the upheaval and sinking of the surface and by the formation of the valley systems due to dislocations of the crust (see Plate). On all these questions scientific opin- ion had become more definitely crystallized by 1921, through the further discussions during the previous decade.

The Origin of the Earth. The origin of the earth may be learned either from study of its composition and structure or from cosmogony and its analogy to the stars. Only a small proportion of the earth's materials are available for direct ob- servation, but indirect methods have demonstrated three facts about the inaccessible interior. It is hot, heavy, and consists of zones having very different physical properties. That the interior of the earth has a high temperature is shown by hot springs, by the warmth of deep mines, by the molten condition of the lavas that are raised to the surface through volcanoes, and by the evidence that all deep-seated rocks have been either melted or re-crystallized under great heat. The temperature below the surface rises at a rate which is known as. the geother- mic gradient. The average rise has been often estimated as about i F. for every 53 ft. of descent. The high temperature of some Queensland hot springs would, unless their water came from a much greater depth than the layers reached by the wells, indicate a gradient more than twice as fast. On the Rand goldfield, on the other hand, the rate may be five times as slow, for at the Village Deep mine it is only i F. for every 253-9 ft. of descent. The rate of increase is naturally quickest near active volcanoes, and it varies in accordance with the geological history of an area and the composition and thermal conductivity of the rocks. The famous observations at the Rose Bridge colliery at Wigan showed that the geothermic gradient varied from i F. for 33 ft. to 57-7 ft.; in the upper part the rate was i F. for every 57-7 ft., in the lowest part the average was i for 48-2 ft. A persistent increase of i F. for 50 ft. would amount to approximately 100 per m.; so the temperature would be at 2,000 F. at the depth of 20 m., and at 100 m. deep would be of solar intensity. It is, however, probable that the material of the deeper parts of the earth is a better conductor of heat than the rocks of the crust; so the geothermic gradient may become more gradual in depth and below perhaps 40 m. the temperature may be almost uni- form, and 2,000 F. may be the highest temperature within the earth.

The second certain fact about the inner earth is that its mate- rial is much heavier than the rocks of the crust. The average specific gravity of the crust is about 2-5, whereas that of the earth as a whole is about 5-7. The heaviness of the internal material may be due either to its condensation by pressure or to its composition largely of heavy metals. That the latter ex- planation is correct is indicated by the evidence of meteorites, of radioactivity, and of earthquakes. Meteorites are fragments of heavenly bodies which fall on to the earth from outer space; they show that not only do the extra-terrestrial bodies consist of the same chemical elements as the earth, but these elements compose the same compounds and mineral species, which are grouped into the same kinds of rocks. Meteorites are divided into two groups: (i) stony meteorites or aerolites, which are composed of such minerals as olivine and bronzite, that are

found in the rocks of the earth's crust, 1 and (2) iron meteorites or siderites, which consist mainly of an alloy of iron and nickel.

That the earth includes a great central mass of metallic material similar to that of the iron meteorites is indicated by its high specific gravity, which is explained by the earth consisting of a heavy core, the barysphere, surrounded by a lighter stony shell, the lithosphere. 2 The specific gravity of the ordinary iron meteorites is about 7-75, so that the earth's specific gravity of 5-7 indicates that the metallic exceed the stony constituents.

The comparative thinness of the lithosphere is also indicated by radioactivity. The earth is radioactive to an extent that can be explained by the limitation of its radioactive minerals to a shell of about 40 m. thick. Among the few materials that are not radioactive are the iron meteorites. Hence the radioactive phenomena of the earth are consistent with its structure as a stony shell of about 40 m. thick surrounding a core of nickel-iron.

This conclusion was first suggested from the propagation of earthquake waves. The late Prof. Milne was thereby led to the conclusion that the earth consists of a rocky shell about 40 m. thick, and of a denser, more rigid core, composed of a material which he called "gelte," and which he regarded as mainly com- posed of nickel-iron, like the iron meteorites. Later study of the distribution of earthquake waves by Dr. R. D. Oldham has shown that within the barysphere is an inner core, which occupies about a fifth of the earth's diameter and does not transmit earthquake waves. This centrosphere in that respect behaves like a gas.

Various lines of evidence therefore together indicate that the earth probably consists of a centrosphere which may be gaseous; of an intermediate layer, the barysphere, which forms the bulk of the earth and consists mainly of nickel-iron; and of an outer shell, the lithosphere, which is probably about 40 m. thick, and forms the rocky crust of the earth. This structure would be the natural result from the condensation of a swarm of small heaven- ly bodies with the average composition of the meteorites. How the swarming may have happened we have to inquire from cos- mogony, which presents two rival hypotheses: the nebular theory attributes the earth to a cloud of white-hot gas, and the meteor- itic theory to a swarm of cold meteorites.

According to the nebular theory of Laplace the solar system was originally a cloud of incandescent gas that extended beyond the orbit of its outermost planet ; as this mass cooled and contracted the mat- ter collected into rings, like those around Saturn or of the Ring Nebula in Lyra. All the matter in each ring was gradually collected into a planet and its satellites, which continued to revolve around the sun along the circle occupied by the ring from which they were formed. As the outer zone of the nebula cooled first the central mass has remained the hottest, and thus the sun is nearest to the con- dition of the original nebula. This theory brought into one logical and consistent scheme so much observational material that it was almost at once accepted, and for a century and a half dominated speculations on the history of the solar system and interpretation of the heavenly bodies. Further observation by improved telescopes discovered important facts consistent with the theory. The nebulae include bright compact clots indicating local concentrations of material, and also empty spaces, such as the Eyes in the Owl Ne- bula. Many nebulae rotate, for the rays of the spiral nebulae are bent backward; nebula M 101 is estimated to rotate once in 85,000 years. Further coincidence with the requirements of Laplace's theory is that the nebulae are not spherical but disc-shaped ; that in Andromeda, being seen obliquely, appears elliptical, and that in

1 The elements proved by Sir William Crookes (Phil. Trans., 1918, vol. 2I7A, pp. 42730) in the stony meteorites include iron, chromium, magnesium, nickel, silicon, sodium, manganese, potas- sium, aluminium, and calcium, in addition to oxygen, hydrogen, and carbon; so that they consist of the elements which form the bulk of the earth's crust. The proportions of the four chief constituents of many stony meteorites are so similar that Sir William Crookes (ibid. p. 426) suggested the possibility of their all*having been derived from the disruption of one planet intermediate between Mars and Jupiter, while he suggests that the nickel-iron meteorites were of a different origin or derived from the core of the same planet.

1 The mineral species found in aerolites are characteristic of the basic rocks; but the existence of acid meteoritic material has been claimed, from the obsidianites found chiefly in Australia. These obsidian buttons have forms found also in fused flue-dust ; their microscopic structure suggests that they are due to the fusion of dust by lightning during dust storms, and that they are aerial fulgurites.