Nature and Bearings of Isostasy. 281 



Abt. XX. — The Nature and Bearings of Isostasy;* by 

 Joseph Bakrell. 



The greater part of thinking in the field of dynamical 

 and structural geology is based upon the evidences re- 

 vealed in the exposures of the rock outcrops or their 

 syntheses as expressed in the geologic map. Yet these 

 structures are of a small order of magnitude as com- 

 pared to those larger structures which underlie the 

 grander features of our planet. Important though these 



* A non-technical summary of six lectures delivered before the Geological 

 Department of Columbia University, March 28-April 13, 1916. This sum- 

 mary was probably written shortly after the lectures were given, but has not 

 as yet been published. The lectures rest in large part upon a series of 

 papers published by Professor Barrell in the Journal of Geology in 1914 and 

 1915 under the general title of ' ' The Strength of the Earth 's Crust. ' ' 



The synopsis of these six lectures ' ' On the Nature and Bearings of 

 Isostasy, ' ' as published by Columbia University is as follows : 



March 28. — The larger relief of the earth. Continental platforms and 

 ocean basins: plateaus and deeps: folded and overthrust mountain ranges. 

 Contrast with Moon and Mars. Views of Dana, Fisher, Suess, and Chamber- 

 lin on the origin and maintenance of the larger features of the earth. 

 Fundamental nature of the problems. 



March 30. — The geodetic evidence of isostasy. Contributions of Pratt, 

 Airy, Dutton, Putnam and Gilbert, Hayford and Bowie. Astronomic and 

 geodetic coordinates and resulting deflection residuals. Gravity measure- 

 ments and gravity anomalies. Differences of these according to hypotheses 

 used. Sum of least squares of residuals as a test of hypotheses. Eesults 

 of the application of the theory of isostasy to geodetic measurements. 



April 4. — The regional distribution and incompleteness of isostasy. Evi- 

 dence from erosion and deposition; from continental glaciers; from group- 

 ing of deflection residuals and gravity anomalies. Magnitude of the loads 

 due to variations in crustal density not in accord with topography. Power 

 of the geodetic method for future geologic investigation. 



April 6. — Evidence of isostasy on distribution of strength. Stresses exist- 

 ing as a consequence of isostasy and mode of readjustment for maintenance 

 of isostasy. Incompetence of isostatic movements to produce folding or 

 overthrusting. Stresses produced by harmonic excesses or defects of mass. 

 Geological processes which lead to an adjustment of stresses to fit the 

 distribution of strength. The curve of strength for lithosphere and 

 asthenosphere. 



April 11. — Physical conditions controlling the nature of lithosphere and 

 asthenosphere. Distinctions between rigidity and strength. Physical con- 

 ditions in a shell of weakness, — the asthenosphere. Adjustment to evidence 

 from tides and earthquakes. Eelations to igneous activity. Eelations to 

 radio-activity and length of geologic time. 



April 13. — Bearings of isostasy on origin of continents and mountains. 

 Elimination of the tetrahedral hypothesis. Continents not built up by 

 weathering during earth growth. Difficulties confronting Fisher 's hypothesis 

 of the origin of ocean basins through the separation of the moon. Difficul- 

 ties of the hypothesis of continental fragmentation. No final solution yet 

 given. Crustal shortening the result of centrospheric shrinkage. Eelations 

 of epeirogenic and orogenic movements. 



