GEOLOGICAL PROBLEMS.

Otago Daily Times, Issue 19000, 24 October 1923, Page 5

GEOLOGICAL PROBLEMS.

ADDRESS BY DR HENSON. At a recent meeting of tho University Club Dr Benson gave an account of the geological work at the recent Pan-Pacific Congress in Australia which he attended. The lecturer began by outlining the origin and general work of the Congress, and proceeded to deal with the purely geological matters in which ho had been specially interested. Though the geologists considered over a hundred papers, a general theme ran through most of them so aiat it was possible to group them, into a single summary though they touched on many phases of gological science. The great problem at tho back of all discussions was the geographical unity of the Pacific lands not only in the present but alsp*in tne past. This unity is expressed jn many ways; in the attitude of tho earth toloa in the marginal lands as regards the ocean on tbe one hand and tho continents on the other; the more or less definite contemporaneity of the successive epochs, depositions of marine sediments or periods of crust folding; in the different countries in the similarity of age, composition, and mode of occurrence of the igneous rocks in tho marginal lands on the one hand ami the oceanic islands on the other; in the special features of the existing volcanoes, strung ns they are along the border of tho Pacific basin from the Alcension Islands to Japan, the Philippine, New Guinea, New Hebrides, and Now Zealand; in tho similarities and differences in the flora, and fauna, marine or terrestrial about the Pacific basin in present and past time as indication of the former continuity or isolation of the various marginal amis, in the similarity of the coastal land terms in the circumpacitic region which give evidence of a like geographical mstory throughout withiiy comparatively recent times, showing uplift or depression of tho coasts, with important effects on tho distribution of coral reefs; these and many other features point to the general unity ot the whole oceanic area. The Pacific Ocean is the grandest geographical feature in the world, and the solution of the problems involved m tracing its ancestry would take ns far back into the studv of the mmesis of tho world itself. This was the great task the geologists at the Congress set themselves to attack. Needless to say J was not solved; the work of generations of students will be required for its solution. But congresses such as these do what no single student can in the way of bringing together and sifting through facts gained in many lands, seeing their possible correlation, picking out promising lines for further research, and giving the individual workers the stimulus of friendly association with those attacking the same problems in other lands. It was customary at tho congress to devote a single day or half day to each topic, such as the various features mentioned above. After a brief introductory exposition of the problem by sSino outstanding authority there was a rapid succession, ot about a dozen torse statements from the delegates of the information bearing on the problem in hand in the particular country represented by each, after which such time as remained was devoted to a summing up and discussion of the facts that hud been presented. The last, however, would not lead very far at a single session of the congress, for the mind couldi not assimilate •incl consider adequately in tho short fame available the vast amount of new material that had been given to it in such highly concentrated form, Tho future congresses however, will build on tho records of the former meetings and follow up the many pregnant suggestions, or tentative generalisations that have been thrown out from time to time. The first day was devoted to a series of statements concerning the younger formations all around the Pacific, and the West Indies. In each land where wo see marine beds and fresh water formations alternating, we recognise the record of tho sea’s advance over margin of the continents, the formation of sediment in this drowned strip, the backward movement of the sea, laying bare those same sediments, which become channelled by rivers, or covered with gravel or lacustrine deposits in which may be entombed the plants and animals that inhabit the land. Thus by the double standard of marine, and freshwater or land organism we seek for moans of dating as accurately as possible tho fluctuations of the sea level. Is it the case that the floodings of tho sea on to the land are simultaneous around the Pacific, or is a retreat of the sea in one area accompanied by a compensating advauoe on to the land in another.'' Or, again, is it possible that the two relations may hold sometimes one, or sometimes the other? The answers to these questions are of great importance in the understanding of the mechanism of earth movement. So it is necessary that wo should have a means of finding as exactly as may be the relative age of formations, and for this the remains of tho rapidly evolving land mammals form a much more delicate test of the age of the sediment that contain them than do those of the more slowly changed, and less specialised marine shells or land plants. To obtain a fullest record of the evolution of the land mammals we must go away from the coastal region, where the terrestrial record is interrupted by incursions of the marine, to the heart of the continents from which the sea has been absent since tho dawn of mammalian life. Biologists have long looked to the highlands of Central Asia as the place where great evolution of mammals took place, the ancestral home of races that have l migrated far and wide. To test the truth of this, the American Museum of Natural History sent an expedition into tho Gobi desert. Financed, organised, and staffed with American thoroughness, it covered over 3000 miles of desert previously inexplored by scientists, and returned with a groat wealth of votebrate remains in which the succession may be traced from the period when the giant reptiles flourished through various stages of mammalian evolution up to the present, though as yet only a preliminary study of the collections has been possible. The story of this scientific adventure was put before the Congress by Dr Hovey, of the American Museum, who will shortly visit this city. From the bordering lands of this central continental region Saghalien, Japan, Java, Burma, and tho Himalayas came now details of the alteration of marine and fresh water beds (with their mammalian remains) which told of the oscillations of the coastline (or of tho sea level), and the last paper dealt especially with the crust-movements leading, up to the building of the Himalayas, which occurred geologically speaking quite a short time ago. Other information on the same lines came from Victoria, and from Central America, and thus gradually our knowledge of the relative age of tho earth movements is increased.

Now, why this desire so carefully to date crust movements about the Pacific? This; If it can bo shown the movements in one region have been repeatedly part and parcel of a system of simultaneous movement all round the Pacific, there is evidently a genetic connection between them. They must arise from a common cause, a force developing within the ocean basin itself leading to a crumpling of the flexible ring of the earth’s crust which surrounds it, and separates it from the more rigid continental masses. And to explain this we get back to something fundamentally connected with the nature and origin of the world itself. What it is wo do not yet know precisely, but an idea was thrown out by Professor Cotton, of Sydney, that will surely receive much attention in the future. To explain it we take for consideration the swinging of a clock pendulum, at first sight quite unconnected with anything geological. The time of the swing of pendulum depends on its length and the pull of the earthgravity. If we take a clock to a portion of the earth’s crust which is underlain by a vast mass of heavy rock the strength of gravity would be greater than normal, and the pendulum would beat more quickly. On the other hand, if under the surface of , the earth was a largo mass of abnormally light rock, gravity would be less than normal, and the pendulum beat more slowly. By this moans it has been ascertained that the Pacific Ocean is underlain by notably heavier rock than underlies the surrounding continents, and its continued existence results from its constant sagging down, while the continents are buoyed up by the plastic yielding of the thus unevenly loaded interior* portions of the earth. But further we know tho earth is not still, but rotates. Now a heterogeneous body that rotates remaining in equilibrium, must have its heavier, and lighter parks adjusted to certain positions or distances from the axis, which depend on the rate of rotation. But this rate is not constant for the earth. The daily course of the tides under the attraction' of tho moon acts as a vast brake which slows down the rotation. Gradually, then, j the adjustment of the various dense masses j within the earth becomes more and more \ out of equilibrium, and vast strains are | sot up until the earth’s rigidity is overcome and readjustment of its several portions, occurs, with consequent warping and wrinkling of its surface. The periodic readjustments of the great, heavy masses beneath the Pacific are on this hypothesis the cause of the successive warnings and dis.lo-

cation of the earth’s surface about its margin. Such mas the new suggestion put before the congress. Resides this explanation of folding on the margin of the Pacific, there are two other hypotheses of the origin of crustmovement as a result of its unequal weighting which arose directly or indirectly. The first was conceived first in the study of the Himalayan survey, and that meet- | ing was fortunate in having to expound it Sir Gerald Lcnnox-Conynghnm, recently head of the Geoclcke _ Survey _of India. Think of a carpenter’s plumb-line. It hangs vertically downward because Us bob is drawn by the earth’s gravity towards its centre. Now. if the earth were homogeneous, a. plumb-line hanging at the foot of the Himalayas would not nang vertically downwards, because in addition to the attraction towards the earth’s ’cntre there would be a sideways pull _ exerted by the mountains on the one side, not counterbalanced by any attraction n the other direction where only the broad plains of the Ganges Valley stretch away for very many miles. Actually, however, the reverse is (he case. Instead of oeing pulled towards the mountains, the plumnbob is pulled away £i’oiU them, afld tllO only reasonable explanation is that the crust of the earth beneath the low iand is dense and acts more strongly on the plumb-bob than docs the lighter though more bulky mass of rocks that makes up I the Himalayas and their foundation. I hus I again we have an equipoise of heavy lowj land and lighter highland, just as we had of heavy ocean floor and lighter continent. A new factor now appears, rho myriad torrents from the great range carry yearly a vast mass of soil, gravel, and huge boulders down from the mountains, and deposit them on the foothills and tne plain. The mountains thus gradually lose their weight and a load is put on the lowlands. At first sight we might say tnat as the equilibrium has been disturbed by the removal of load, the loaded plains must sink a little deeper, and the unloaded mountains bo thrust up a little higher to restore the balance as soon as the strain set up is sufficient to overcome the earth s rigidity, and so the mountains are perpetually renewed. So, indeed, ran the reasoning in the earlier stages of the _ hypothesis. But would wo not be mistaking a very superficial effect for the true cause, which may be much deeper. The second hypothesis recently announced suggests that it does so. Any portion of the earth s crust is made up of numerous elements, grouped into various compounds called minerals. It is often possible to group precisely the same amount of the different elements into diverse combinations or minerals, one grouping favoured by one set of conditions another by another. Now just as a regiment would occupy a smaller or larger parade ground, according to the way its units were grouped, so while remaining of precisely the same total composition chemically, rocks may be so changed as regards the grouping of their minerals that under one set of conditions tney may occupy comparatively little space or bo very dense, while under other conditions they may be more bulky and consequently less dense. If, then, conditions were to change within the earth, it is conceivable that the rocks of the heavy crust under the present lowlands might reerystallise into their lighter more bulky form, and the rocks beneath the mountains into their denser less bulky form. If this were to happen, the subacrnstal swelling in one region and contraction in the otner would result in the elevation of the earth « surface on the one hand, its depression in the other. In this hypothesis, the continued growth of the Himalayas by tne expansion of the rocks below, and the continued deoression of the Gangehe plain, by the contraction of the rocks below na« made possible the continuous washing down of the gravel, etc., from the mountains to the plains, which is thus a minor effect, and not a prime cause of the mountain growth. This conception of mountain growth must come as a modifying factor not as an alternative to that suggested by Professor Cotton. Related to these great movements are the features of igneous activity, both deepseated and superficial. Rocks within the earth are extremely hot, above their normal melting noints, but kept solid by the huge pressure‘to which they are subjected, but when during earth-folding that pressure is relieved, some portion of the crust may become molten, and so free to bo squeezea hither and thither through the crust until it cools and solidifies either within the crust, as great masses of plutonio rook, granite and the like, or after having been ejected through vents or fissures it is erupted from the surface and flows out over uie land, or sea bottom, or piles up great cones of volcanic ash, the result of explosive ejection. Thus around the mobile mHrpfiii of the Pscihc "\vo find the most marked zone of volcanic activity now in existence, while rents and fissines of bather different origin cause the volcanoes of the islands dotted about within the Pacific area. These, too, received discussion 'll the Congress, and great interest was displayed in the splendid work of the volcano logical survey of the East Indies, and that being carried cn near Honolulu, in the crater of Kilanea. But a very special interest attaches to those portions of molten rocks which consolidated within the earth, and were not connected with volcanic outbursts. A volcano erupts explosively because the molten rock it ejects contains a largo amount of gas, water-vapour, etc., which must expand when able to do so. No such free expansion is possible for the vapours and fluids given off from the molten rocks as they solidify within the earth s crust, but they carry in solution tho metals that were distributed through the molten rock and penetrating into the fissures in the earth’s crust and they deposit the valuable ores. Hence tho origin and age of tile metalliferous veins around the Pacific are intimately connected with tho structure and origin of the Pacific basin itself, and that of its surrounding land masses. A brilliant generalisation concerning this problem was put forward by Mr Andrews, Government geologist of Now South Wales (author of the recent magnificent volume on tho Broken Hill region), and was one of those sweeping generalisations which by reason of the very exceptions that spring to the mind, call for immediate inquiry and discussion.

Tbo re is, however, yet another effect of the forces of earth-folding of the importance of which there has been recently a terrible manifestation. The growing strains in the crust, as various portions get increasingly out of adjustment, result in little fractures and slippings of the rook musses over one another that are expressed on the surface by destructive earthquakes. Now the new advances in the study of these are two fold. The first put before the congress by the veteran Japanese authority, Professor Omori, concerned itself with the study of the minor tremors and tilting of the earth’s crust by which the accumulation of stresses could bo detected and the prediction of major -earthquakes and volcanic eruptions could be made with some confidence. The great utility of these studios in the' saving of human life was shown by one dramatic example—the removal by Government authority (warned of an approaching eruption) of 30,000 people from one of the Japanese islands three clays before the volcanic outburst destroyed nearly every building on it. The second lay in the fact that every such shock sets up vibrations radiating in all directions through the earth, or along its surface; and differing in some degree in speed and nature according to the materials which they traverse. These vibrations may pass under oceans or continents, or even right through the centre of the globe, and are recorded by delicate instruments termed seismographs, which may be situated thousands of miles from (he site of the earthquake itself. These records then afford indirect means of ascertaining the nature of the interior of tire earth, and its variation or heterogeneity and lend support especially to the conclusions that there is no general molten interior of the earth, which except for local and temporarily molten masses is solid throughout, and further that the portions of the crust beneath the oceans are more dense than those beneath the continents just as the swinging of the pendulum had indicated. Hero, too, the congress was favoured by the presence of distinguished authorities from Japan, the Philippines. Java, and Australia, who expounded before it- their researches in this the newest of the earth sciences.

Essentially related to all those problems though at first sight perhaps totally distinct were two other mailers in which research has been very active, and the data concerning which were discussed before the congress. These were the problems of the last great Ice Age, and that of the origin of coral reefs. In all lands around the Pacific, oven in l New Guinea where mountains rise to sufficient height, there is evidence that in the not very distant past extensive glaciers existing filling the valleys in the high mountains or even debouching on to the lowlands (such, for example, as the Clinton Valley glacier, which broadened out into an ice-sheet filling ihp To Anau basin). Was this refrigeration brought about by an upthrusting of the land masses into the higher, cooler layers of the atmosphere, and thereby deflecting the normal circulation of planetary winds? Such elevation would necessarily Ire accompanied by the increase of land areas, and the lifting above sea level of isthmuses connecting formally

separated land masses and permitting the iutonnigration of plants and animals. Or, on the other hand, was the cooling brought about by some fluctuation in the supply of heat received by the earth from the sim, which like other stars may bo variable? The latter is the view to which American and Australian thought seems to lie tending, though up to the present the British and New Zealand geologists have inclined more to the former hypothesis. Again, was the oncoming and retreat of the ice due to a single climatic fluctuation or wore there here, as in the northern hemisphere, a succession of several closely following periods of cold separated by warmer spells. Tho evidence for this would he found in the study of land forms, partly ■a tho distinction between the broad valleys smoothed by the slow-moving glaciers, and the gorges notched in them by the dashing torrents during the intervening periods of warmth, and partly in the succession of layers of boulder-olay or gravels laid down by the successive glaciers and the streams which drain from them.

Again, the withdrawal of water from the sea, when in the form of ioe-sheets it was accumulated on the land, must have meant a lowering of the sea-level even in tropical regions, with a great effort therefore on all the geographical and biological features of tho sea shore. Darwin, it will be recalled, explained the origin of coral reefs by the gradual upbuilding of the coral about a sinking coast, and thought the abundance of tho reefs in the Pacific was duo to a general subsidence of the ocean floor. But may not this apparent subsidence be really due to tho rise in sea level as the great glaciers melted and gave back their imprisoned water? What significance shall we put. for example, on the reef-dotted, shallow South China Sea? The floor of this is evidently an old lowland plain, which once connected the Malay Peninsula, Sumatra, and Borneo, and has now been drowned to just the depth (30-40 fathoms) that theory would lead us to expect would be equal to the rising of the sea-level as the ice melted. Or, again, what significance shall wo attach to those Pacific Islands that show signs of instability, not merely of subsidence, but of some elevation, and tilting? Perhaps, after all, the problem of the origin of coral reefs is not so completely solved as may have been thought, and other factors than those considered by Darwin mav bear importantly on the problem, though it was satisfactory to find a full accord with the correctness of Darwin’s explanation in many cases and notably that of the Funafute Atoll, where some 25 years ago Professor Sir Edgeworth David had put down a bore for 1114 ft, finding all the way down shallow water types of coral standing upright in the position of growth. ~ , Such were some of the questions discussed at the Pan-Pacific Congress. Space would fail us to describe the other matters dealt with, the present and possible oilfields, and the other mineral resources, or the detailed work of the official geological surveys in the lands around the Pacific and other matters of more academic interest. Suffice it to say that interest was quickened and advances recorded in nearly every branch of geological science, and the work of teacher or investigator alike throughout the countries represented must be increased in value as a result of that most important congress.