Ransome. I 



The Great Valley 



403 



Dana, in the last edition of his "Manual of Geology" (1895),* 

 points out that the sinking of coast-lines is often in excess of what 

 would be demanded were the cause of sinking merely the accumu- 

 lation of sediments. He leaves the theory practically open, how- 

 ever, and, like Geikie, devotes less attention to it than its interest 

 and importance seem to demand. 



The foregoing sketch, although not an exhaustive history of the 

 theory of isostasy, has yet aimed to take note of all the more im- 

 portant utterances which have been published upon the question. 

 No attempt has been made, however, to follow up the various sug- 

 gestions and hypotheses which have from time to time been advanced 

 as to the effect of masses of ice and snow upon the earth's crust, 

 particularly during glacial times, among the most startling examples 

 of which may be cited the hypothesis of Prof. Alexander Winchell.f 

 in which the great volcanic outflows of the western states are 

 regarded as having been squeezed out during Pleistocene times, by 

 the weight of a continental ice-sheet some 5,000 feet thick. Such 

 deductions have generally been marked by a more than ordinarily 

 hypothetical character, owing to the great difficult}- in the way of 

 securing" reliable data. The supposed masses, owing to their small 

 specific gravity, must have exerted a relatively slight pressure upon 

 the earth's crust, or, in other words, have constituted but a small 

 departure from isostasy as compared with the sediments laid down 

 in the past, and still covering vast areas of the earth's surface. If 

 the deposition of these, and the maintenance of their present posi- 

 tion above sea-level, have been factors of sufficient importance to 

 cause an appreciable readjustment of the earth's crust, then we may 

 hope to get from their study some quantitative scale to apply to the 

 supposed effect of a great ice-cap. But if it can be shown that the 

 rigidity of the earth's crust is not perceptibly diminished under such 

 sedimentary loading, then the glacial aspect of the problem will 

 require no further attention. 



On the whole, the foregoing historical outline maybe considered 

 as showing that the isostatic hypothesis as ordinarily understood by 



* PP- 377-379- 



t Pressure of a Continental Glacier, Am. Geologist, Vol. I, pp. 139-143. 

 3 



