I92I. No. II. THE STRANDFLAT AND ISOSTASY. 299 



This vertical movement would be effected without any "flow" of the 

 plastic substratum underlying the rigid crust. If the transformed rocks 

 form parts of the lithosphère the rise of tlie crustal surface caused by 

 their expansion would be addcil to tlie isostatic upheaxal of the crust 

 caused by the denudation. 



Tf it is rocks of the ]:ilastic substratum untlerl}ing- the lighter, rigid 

 crust, which are transformed, they ma\" by their expansion become parts 

 of the lig-hter crust, and will then increase the upheaval of the latter caused 

 by the denudation. In that case the thickness of the lighter crust would 

 not be much reduced by denudation. 



If the transformed rocks remain parts of the plastic substratum, 

 their expansion ma_\- reduce the isostatic upheaval of the latter, l)ecause 

 the expansion makes the layers of the substratum lig-hter, and the crust 

 mav sink deeper into it, which would, however, necessitate a "flow" in 

 the substratum. 



If instead of a reduction of the weight of the crust (c g. by 

 denudation) we suppose an increase of its \\eight by deposition of sedi- 

 ment, or bv the formation of an ice-cap, the deep-seated rocks may be 

 inverselv transformed, and a gradual increased sinking of the crustal 

 surface would be caused. 



As, however, we still know mucli too little about the actual c inditions 

 at these depths l^elow the earth's surface and about the possible trans- 

 formations that may take place there, we must leave the crustal movements, 

 thus caused, out of our present considerations. 



If we trv to form an idea of the isostatic movements of the earth's 

 crust caused l)y a load, c. g. by an ice-cap, the simplest supposition is 

 that the crust is floating on a semifluid or viscous, molten magma, in a 

 manner similar to that in which an ice-sheet floats on water. In that case 

 the conditions may be considered as being practically hydrostatic below 

 a certain depth. /. c. there is practically a uniform pressure in all direc- 

 tions, and when the crust is depressed by an additional load, it will 

 gradualh' sink down into the magma, in a manner similar to that in which 

 a loaded ice-sheet will bend under a load and sink deeper into the water, 

 until it attains its new level of equilibrium. The plastic magma under 

 the sinking crust will be grarlually pressed out towards the sides, and an 

 "undertow" will arise. This will continue till the crust has reached its 

 new level of equilibrium. The volume of the magma displaced sideways 

 by the "undertow" will be e(|ual to the total volume of the depression of 

 the crust below its initial level, the possible changes in the volume of the 

 magma caused by pressure not being taken into consideration. 



We do not know what the state of matter may be at the depth of 

 the zone of compensation, perhaj)s at 120 kilometres, or more, below the 

 earth's surface — whether it is .solid, viscous, fluid, or some other state 

 unknown to us. The experimental researches by \\ I). Adams [1912, 1917] 



