278 G. R. MANSFIELD STRUCTURE OP ROCKY MOUNTAINS 



compression in different regions where moimtain-building has occurred. 

 Thus in the island of Angiesea, whose highly complex geology has been 

 studied in great detail by Greenly,^® the accessible part of the Holyhead 

 recumbent fold is said to have a real horizontal amplitude of about 60 

 miles. Great thrust-faults are also present. It is aj^parent that in 

 Angiesea' the compressive forces were more intense than in southeastern 

 Idaho. 



IsosTATic Features 



In the Alps, as described by Heim,^^ the gravity defect is at a maximum 

 under the line where the "decke" folds of relatively lighter rocks were 

 originally piled highest. It is normal at the south foot of the Schwarz- 

 wald to the north and in excess at the top of the Schwarzwald. Similarly^ 

 near Locarno, at the south, there is another area where gravity is in 

 excess. This is the belt where the roots of the folds that now form the 

 "decke" are supposed to lie and where basic eruptives are now heaped up. 

 The surface of normal gravity lies unsymmetrically beneath the Alps, its 

 gentler and longer slope passing upward toward the north. The folded 

 complex of the Alps is thus believed to float with its deeper parts con- 

 cealed in the earth's crust, much as an iceberg floats in the sea. This 

 attitude is produced by sinking, in consequence of the overload due to the 

 compression of the crustal mass. So it happens that the Alps are not 

 5 to 10 times higher. 



A different condition seems to obtain in southeastern Idaho. On the 

 map of gravity anomalies, published a few years ago in Gilbert's paper,^^ 

 southeastern Idaho is shown to possess an excess of gravity, while central 

 and northern Idaho and adjacent parts of Montana are deficient. These 

 last named regions are areas of batholithic intrusion. Under the isostatic 

 theory, batholithic intrusions might normally be expected to occur in 

 regions adjacent to areas of intense folding and overthrusting. This is 

 apparently the case in the Alps. Since folded sedimentary rocks are 

 relatively lighter and intrusives supposedly heavier, the plus anomalies, 

 representing excess of gravity, should, under the hypothesis, lie in the 

 batholithic areas and the minus anomalies in the sedimentary areas.. 

 This again is apparently the case in the Alps, but the reverse actually 

 happens in southeastern Idaho. The Snake Eiver lava plains lie in the 

 area of excess gravity, and the weight of the basalts that underlie these 



29 Edward Greenly : The geology of Angiesea. Geol. Survey Great Britain, Mem., 1919^ 

 p. 181. 



«" Albert Helm : Geologic der Schweiz, Bd. II, Erster halfte, Leipzig, 1921, pp. 52-56. 



31 G. K. Gilbert : Interpretation of anomalies of gravity. U. S. Geol. Survey Prof. 

 Paper 85, 1914, pp. 29-37. 



