2-JUl: W. H. HOBBS THE ASIATIC ARCS 



blanketing slide or nappe, there appeared the first Yolume of Suess's Das 

 AntJitz der Erde, in which the j^regnant idea of the nnsYmmetrical 

 mountain arc was shown to have an almost universal application. Over- 

 coming the most stubborn opposition, the idea of one-sided (einseitigen ) 

 as opposed to two-sided pressure (ziieiseitigen Dru-cl-) at length made 

 converts, even of its enemies, notably Lugeon and Heim. who are today 

 the foremost expositors of the doctrine of one-sided compression. 



Views of Xobee 



Ignoring all the evidence derived from the plan of the arcs, it has 

 remained, strangely enough, for a Viennese student of mountains, Leo- 

 pold Kober, to revert to the now long-abandoned idea of symmetrical 

 bilateral disposition of the compressive mountain-building forces. The 

 Alps are by Kober connected up to the Atlas Mountains in Africa as 

 though they comprised a single folded area, and the well-known pro- 

 files have been so distorted as to make a supposed folded zone, about one 

 thousand miles across, extend downward into the earth's shell a distance 

 of some 300 miles, so that horizontal compression might appear to get 

 enough "Tiold" to compress the lens — ^but what Kober draws as a wedge — 

 of sediments, on which the forces are none the less made to act at a very 

 acute angle. Such a symmetrical distribution of forces he then by an 

 even greater distortion of the known profiles has made to apply to most 

 other folded mountain ranges. 



There are four well-known regions, and four only, where hy extending 

 from one range to another across hroad intervening non-mountainous 

 areas, as Kober has done, such a bilateral symmetry can be made out. 

 Three of these are on the site of the former Sea of Tethys and its exten- 

 sions — what Emerson has called the zone of the intercontinental sea - 

 and the author the twiu zone of the earth.^ The fourth example is that 

 of the South Shetland Islands, lying between the southern apex of South 

 America and the Antarctic Continent. 



It may fairly be claimed that among students of mountain formation 

 the idea of "one-sided'* as opposed to •*two-sided'* compression has re- 

 ceived general acceptation, though for correct terminology- we should say 

 that what the idea involves is really an excess of lateral compressive stress 

 from one side of the folding range, which results in actual movement of a 

 portion of the rock layers from the side of the greater stress toward that 

 of the lesser. 



- B. K. Emerson : The tetrahedral earth and the zone of the intercontinental seas 

 « presidential adressi. Bull. Geol. Soc. Am., vol. 11. 1900. pp. 61-96. pis. 9-14. fig's. 1-7, 

 3 W. H. Hobbs : Earth evolution and its facial expression. Macmillan, 1922. p. 92. 



