Br. A. Morley Davies — 07i Isostasy. 125 



the water, perhaps due to the washing in at spring-tides of waves 

 from the open sea, now almost ready to break into the landlocked 

 area in which the Keuper Marl was formed. It is interesting to 

 note that, north of Nottingham, Tea-green strata also occur, at the 

 bottom of the Keuper, unconformable to the Bunter below, and 

 perhaps repz'esenting the last traces of brackish water before the area 

 was finally cut off from the open sea. 



V. — A Note on Isostasy. 

 By A. MOKLEY Davies, A.E.C.S., D.Sc, F.G.S., Imperial College of Science 



and Technology. 

 ri"lHEK.E is a regrettable tendency to looseness of thought among 

 X geologists on the subject of isostasy. I give no quotations in 

 support of this assertion, firstly because, being always in the form of 

 casual allusions to the pi'inciple, they would require long hunting 

 down ; and secondly because, if given, they would fasten npon a few 

 individual geologists a criticism which should be more general. 

 The usual form in which the looseness of thought shows itself is in 

 explanations of shallow-water deposits of thickness greater than 

 their depth of accumulation. We are frequently told that such 

 thick deposits result from local subsidence due to the loading of the 

 sea-floor by the great weight of sediment, and reference is made to 

 the principle of isostasy as justifying this explanation. 



The principle of isostasy is that the distribution of mass in 

 a heterogeneous earth tends to be so adjusted that variations of the 

 surface from the theoretical ellipsoid of rotation are compensated by 

 differenaes of density in the deeper parts of the underlying crust. 

 The continents are supported on a mass of less density, the oceans on 

 a mass of greater density; and similarly for the mountain-chains and 

 ocean-troughs in relation to the average of the continents and oceans 

 respectively. 



This compensation of excess of matter at the surface by defect of 

 density below and of defect at the surface by excess of density 

 below is termed isostatic compensation, and the adjustment of the 

 earth's crust towards a condition of isostatic equilibrium caused by 

 gravitative stress is termed isostatic adjustment. Compensation is 

 supposed to be complete within a comparatively shallow depth 

 (122 kilometres according to the later calculations of the United 

 States Coast and Geodetic Survey), and the mass of matter under 

 any protuberant or depressed area from sea-level down to this depth 

 is termed the supporting column of that area. 



Now imagine the adjacent parts of an ocean and continent, in 

 perfect isostatic equilibrium. Denudation removes material from 

 the continent, which is deposited on the ocean bottom. Isostasy is 

 disturbed, and if isostatic adjustment takes place the tendency will 

 be for the continent to rise, the sea-floor to sink, and material in the 

 depths to "flow " from the supporting column of the ocean into that 

 of the continent. But how far can these movements go ? The 

 oceanic supporting column is composed, ex hypothesi, of material of 

 more than average density; the material deposited, being uncon- 

 solidated sediment, is oi less than average density. The former we 



