26 W. M. DAVIS 



considered here — in which their enormous overthrusts were 

 enormously eroded; and that the present eminence of the Alps is 

 not a residual of their upheaval at the time of lateral compression, 

 but of a much later and more moderate arching — an undulating 

 arching as Penck now interprets it — by a relatively simple uplift. 

 Moreover, the great work of glacial erosion is now fairly well under- 

 stood and carefully allowed for, the best study of this great problem 

 being in Penck and Bruckner's masterly work, "Die Alpen im 

 Eiszeitalter" ; and the immediately preglacial forms of the mountains 

 are believed on valid grounds to have been prevailingly sharp- 

 crested ridges. Finally, instead of assuming a single rapid upheaval 

 as having introduced the present cycle of erosion, Penck proposes 

 several alternative postulates regarding the rate and duration of 

 upheaval and examines their consequences, with the result of select- 

 ing the postulate of long-continued upheaval at a moderate rate 

 as providing the best counterpart of the movement to which the 

 present Alps owe their elevation, and of ascribing their fairly uni- 

 form summit altitudes to a balance between upheaval and erosion. 



Whether the deductions from the postulate of long-continued 

 upheaval are accepted as vahd or not, the general sequence of forms 

 that is traced out is a beautiful one. First, both the altitude and 

 the relief of the young mountains are increasing, the increase of 

 altitude, but not of rehef, being as fast as the upheaval of the 

 mountain mass; then while the ridges are sharpened, their altitude 

 continues to increase, but now a little more slowly than the rate of 

 upheaval, and the relief is held at a constant value; next, continued 

 upheaval being balanced by degradation in full maturity, altitude 

 reaches and is maintained at a constant maximum, while rehef 

 stands unchanged; later, upheaval ceases and altitude is slowly 

 decreased, although for a time the ridges are still sharp and their 

 rehef is still unchanged; finally, the sharp ridges of the quiescent 

 mass are rounded and lowered, and thus both altitude and relief 

 are decreased to smaller and smaller values as old age is entered 

 upon. There is elegance as well as originality in these deductions. 



Extensions and corrections of the erosion cycle. — Evidently the 

 explicit consideration given in the Gipflur der Alpen to several ideal 

 cycles of erosion, differing from each other in the rate and the dura- 



