NO. 31 GEOLOGICAL STRUCTURE OF THE ALPS — WILLIS 1 3 



Let a rock mass start at a and be thrust to a'. Then let a later 

 thrust carry it to a". It will be isolated by two thrust planes and 

 will be superimposed on younger strata more or less nearly over the 

 place from which it started. One has but to think of a person mount- 

 ing a stairway which turns back on itself. The " exotic " masses have 



Fig. 3. — Intersecting thrust planes. 



gone upstairs, some in one direction only, some in two; and the strata 

 have been folded and squeezed into most complex and irregular forms 

 in the successive movements. 



In conclusion I desire to express my appreciation of the able and 

 painstaking investigations of Prof. Lugeon, and to repeat an expres- 

 sion of regret that it is iinpossible for me to accept the theories of 

 structure which have been developed from his studies. 



