FOLDS AND FAULTING. 465 



this year have been submitted to a searching analysis by 

 Dr. Rothpletz (15 and 16). 1 



We do not propose to review in detail the former 

 of these two works, or recite the various arguments he 

 adduces in it, these being more concisely formulated in the 

 second one (read at the late Congress in Zurich), which 

 lead him to conclude that many of the theories advanced 

 to account for mountain building have failed to explain to 

 his satisfaction the phenomena produced; suffice it to point 

 out that, in his opinion, they fail to explain Helmert's re- 

 searches on the diminution of mass under mountain chains, 

 and that most of the effects could equally well be the result 

 of expansion as of contraction, by means of the radial and 

 tangential pressures that would be set up. 



Dr. Rothpletz, however, carefully guarding himself 

 against accepting expansion as the solution, throws it out as 

 a suggestion, seeing that he considers the theory of contrac- 

 tion has failed in one important particular, and that it is 

 necessary to seek other bases more in accord with the facts 

 known to us. 



Summarising the conclusions he arrives at in his latest 

 work, in which he has dealt with the whole question of 

 overthrusts, we find many old views discarded, and new 

 ones suggested. 



On the first point, which will perhaps find very general 

 acceptance, he formulates : — 



(1) All overthrusts are the accompaniments of foldings 

 and mountain elevation, and are the result of the same 

 forces acting more or less horizontally, that is, tangentially 

 to the earth's surface ; and that where tangential pressure 

 is still active, it produces in local and somewhat sharply 

 bounded regions compression and folding. 



(2) That overthrusts in general have a strike closely 

 parallel to the folds, and their clip, though, as a general rule, 

 it is towards the mountain region, may occasionally be 

 from it. In Daubree's experiment (fig. 2) it will be seen 



1 This, pamphlet, read before the International Congress at Zurich, has 

 only just been published, and I have been indebted to Professor Sollas for 

 the loan of an advance copy. 



