MECHANICS OF FORMA TION OF ARCUA TE MOUNTAINS 207 



supported by a strong formation this will not occur, but it will in 

 any case eventually be rotated as a whole about its base and form 

 a normal recumbent anticline. 



Subsequent to the stage of underturning of the anticline, the 

 active force of compression tends to reduce the volume of the 

 included roll of the inferior strata with the effect of stretching and 

 attenuating the under limb of the anticline in the competent mem- 

 ber. Attenuated upper limbs of anticlines, though unknown in 



^Ccx^tn-t i n en"ta.\ 



Fig. 39.— Schematic diagram to illustrate the order of development of mountain 

 arcs from within the series outward, and the development of anticlines within each arc 

 from without inward. 



rock folds, are illustrated by the ice-folds within glacier snouts, 

 where they are due to true overturning movements. 



The partial relief of load from the inferior formations beneath 

 the competent member in the arch may make possible under 

 suitable conditions of temperature a local development of a magma 

 macula; and the tendency to reduction of volume with continued 

 underturning of the anticline supplies an efficient cause of the 

 elevation of such lava toward the earth's surface. Association 

 both in time and in place of volcanoes of the Pacific type with 

 growing mountain arcs would thus be accounted for. 



