382 DYNAMICAL GEOLOGY. 



tion, and adds that the denudation could not have produced any result until the elevation 

 had made some jDrogress. The theory supposes the isostatic condition of the globe ; and if 

 this was the condition in Cretaceous time before the elevation began, the elevation never 

 could have taken place without force from some real source. 



In accordance with the above, the evaporation of the flooded Great Salt Lake (called 

 Lake Bonneville), which, in the middle of the Quaternary era, had reached a depth of 

 1000 feet, has been suggested by G. K. Gilbert as the cause of the inequality of height in 

 different parts of the terrace that marks its old coast-line. The change of level indicated 

 is stated to be about 200 feet. The pressure of 1000 feet of water, or that removed by 

 evaporation, is equivalent to 450 pounds to the square inch. The theory implies a molec- 

 ular transfer (as the waters disappeared in the Middle Quaternary) from the outside 

 region to that beneath the lake. The explanation is put forward by Gilbert with the 

 statement that further investigation is required before the view can be regarded as estab- 

 lished. 



The difficulty with the Gravitation Theory in its best form is that it does 

 not supply the amount of pressure, and of contraction or expansion, which is 

 required by the facts. 



This is true of Reade's theory, even with the recurrent work of igneous 

 intrusions. In the case of the Appalachians the width of the geosyncline 

 from S.E. to iST.W. is less than 250 miles. The ratio of maximum depth 

 to width is about 1 to 40, or that of a trough as wide as this printed page 

 and one ninth of an inch deep. The depth of the strata, 40,000 feet, gives 

 for the temperature at the bottom of the geosyncline (supposing the rate of 

 downward increase to be 1° F. for each 50 feet of descent) 800° F. Conse- 

 quently an expansion of 2-75 feet for 250 miles of width and for each 100° 

 F. amounts to 5500 feet, or a little over a mile. Lesley makes the actual 

 shortening over the breadth of the geosyncline in Pennsylvania, in con- 

 sequence of the flexures, to be 44 miles, and Claypole 88 miles. The dis- 

 crepancy is too large to be removed by questioning either estimate. Many 

 of the single folds would use up several times the 5500 feet. So it is in 

 other cases. 



In the Laramide Range, of southern British America, a thickness of the rocks in the 

 geosyncline of 34,000 feet, and the width of the trough about 150 miles, give for the tem- 

 perature of the bottom about 700° F. ; and the expansion, under these conditions, would 

 be only 2900 feet for the whole width. The displacement horizontally of one of the 

 several faults, according to McConnell, is 7 miles, or nearly 13 times the maximum 

 allowed for the range by the theory under consideration. In the Juras, Helm found the 

 contraction by flexures to be 3 miles, or one fourth, for the distance between Lake Geneva 

 and Saint Claude ; and in the eastern Jura to be 4 miles in a breadth of 7 miles. 



There is the further objection to the theory that in a trough, having the 

 depth only a thirtieth or a fortieth of the breadth, the expansion would act 

 nearly equally in all directions ; so that while longitudinal ridges might 

 prevail, transverse should be common instead of uncommon. But the ex- 

 panding effects from the heat of successive igneous intrusions are to be 

 added, according to the theory, ridges thus succeeding ridges. In the case 

 of the Appalachians, there were no igneous intrusions along the chief part 



