2 I PROCEEDINGS "l PORON DO ME] I tNG. 



column being inversely as its mean density. In accordance with this theory, promi- 

 nences and depressions of the surl ist in virtue of a principle of equilibrium, 

 called isostatic* [Jnder hydrostatic equilibrium 1 1 1 «- surface of a free liquid is level ; 



under isostatic equilibrium the surfa< i* a non-homogeneous solid, capabli ous 



Hi »w. i- une\ 



There are thus two possible explanations of the inequalities of terrestrial surl 

 and ili may bi severally by the terms rigidity and isostasy. 



In connection with :i Btudy of Lake Bonneville, a large body of water temporarily 

 filling a basin "t Utah during Pleistocene time,-) observational data were gathered 

 bearing on the question of rigidity versus isostasy. 



] The Wasatch mountain range is carved from a large block of crustal material, 

 uplifted along a fault plane at one Bide. The block adjoining the fault plane <>n the 

 opp le is thrown down. Erosion is continually transferring material from the 



uplifted block to the down-thrown block, and there is direct evidence that the moun- 

 tain is steadily rising or the valley sinking, or both. Some advocates of the isostatic 

 tl rv would regard this progressive relative displacement as a din of the con- 



tinual transfer of load. Under this view the mountain block has less density than the 



valley block, and the two are in isostatic equilibrium ; the unloading of the untain 



block l>y erosion and the loading of the valley block by deposition disturb th [uilib- 



riuni. and it i- restored by vertical movement on the fault plane. 



An arm of Lake Bonneville occupied the valley, tilling it t<> an average depth of 



500 or 600 feet, and this load of water was somewhat quickly added and afterward 



..•what quickly removed. If the valley block were delicately sensitive t'> the 



application of load, it should bcdepressed about 200 feet by the access of water, and 



should rise a corresponding amount when the water was removed. But this did not 



ir. On tl ntrary, the depression of the valley, as shown by changes occurring 



along the fault plane, continued alike during the presence of the water and after its 

 removal. It is therefore concluded that the local transfer of Load from one orogenic 

 block to the other i- not the primary cause of the progressive rise of the mountain and 

 depression of the valley, and the question arises whether the mountain range may not 

 be wholly sustained in virtue of rigidity. 



idering the main body of Lake Bonneville, it appears from a study of the 



that the removal of the water was accompanied, or accompanied and fol- 

 lowed, by the uprising of the central part of the basin. The coincident f the 



pbeno na may have been fortuitous, or the unloading may have been the cause of 



the uprising. Postulating the casual relation, and assuming that isostatic equilibrium, 

 disturbed by the removal of the water, was restored by viscous Blow of crust matter, 

 then it app bservational data J) that the flow was nol quantitatively 



sufficient t" satisfy the created by the unloading. A stress residuum was loft 



to be taken up by rigidity, and the measure of this residuum is equivalent to the 



of from (00 tbic miles of rook. 



phenomena and theoretic considerations arises the working hypothesis 

 I the m if the crust is a prominenoe or a o »ncavity about 



ill volume. 



i mi \in Joui , Vol. XXXVIII, 



■ ■ •' 1 1 1 ■ i in iho Seoond \ n ii ii nl Report of Ihi otogl- 



1 ut h ill :iii|.«'nr In i« memoir on Laka Bonnei ill » in 



ipni <>( ii 



