EARTH'S STRUCTURE

Evening Post, Volume CXII, Issue 12, 14 July 1931, Page 5

EARTH'S STRUCTURE

THE LITTLE WE KNOW

DEDUCTIONS OF SCIENCE

The formation, composition, and interior condition of the earth were the subject' of the second lecture oi the ' "earthquake series" arranged by the 'Wellington Philosophical Society. It ■was given in, the Dominion Museum last night by Mr. W. M. Jones. Dr. E. Marsden w,as in the chair, and the audience was'again large and was quite inclined at the close o£ the lecture, which was illustrated with lantern slides, to discuss some of the riddles propounded. After discussing the prevailing theories in regard to the beginning of the earth as a separate body, the lecturer pointed out the two main theories, the planetesimal and the tidal, led to different ideas as to the evolution'and present condition of the earth. Our knowledge of what the interior of the earth was like, was based to a large extent on deduction, seeing that an. insignificant fraction, less than ten miles, was accessible to direct observation. The various sciences, however, all contributed information which helped in these deductions. The niain conclusions of the geologists, based on the observed distribution and constitution of the materials exposed on tne surface, indicated the following general sequence, from the surface toward the centre:-^), 'A mantle of sediments, of variable thickness, but not more anywhere; than ten to twenty miles, covering (b) a layer of granite, not S thY forty miles thick, which «, ToweVer, much thinner or absent under the ocean basins;, (c) a layer of more basic material, basalt or some, rock of I similar composition, of uncertain thickS below which is (d) a high-pres-sure fa'cies of the same composition s rh as eclogite, passing downward into (c) a more basic material, such as periof re'ent date, but promised muchjo mmm Topics are. • Deductions as to the tnieK nl*This principle"has received,, fairly geneSaccer^ceV^ofW*Pf.rds'Withthe ideas of a denser stratum the earth-caused by its rotation ' "Astronomy can show, from the theoryiof the 'bodily tide' or the, amount of yielding of the earth.as a whole to the tidal pul s of the sun^ and moon, ana from the existence of a small periodic variation oftho position of the Sdue to the wobbling effect caused by inequalities of distribution of such things, as snow, air pressure, and vegetation, that the earth as a whole has a rigidity of the. same order as ttatj« 'steel This, however, does not deny the possibility of a'liquid core, of con-; ' giderable viscosity."