238 C. R. LOXGAVELL KOBER's THEORY OF OROGEXY 



border ranges with it (figure 4). Parts of both forelands ma}' break 

 down, carrying the entire orogenetie zone, perhaps leaving the tops of 

 border ranges projecting above sealevel as chains of islands (figTire 5). 

 The island chains east of Asia may represent a combination of these 

 two modifications (figure 6). The eastern foreland is covered by the 

 Pacific; a typical foredeep borders Japan; the Yellow Sea is a 

 shallow, partly filled foredeep, and the western foreland is partly broken 



Figure T. — Blocl-diagirntmi cines samt dem Vorlande vevsenJcten Orofjcn 

 Kober's figure 32. page 167. 



down. Again, both forelands and most of. the orogen may be submerged 

 figure 7 ) . Perhaps this type is represented by the island chains of Xew 

 Caledonia and the Xew Hebrides. The suboceanic ridge extending north 

 and south in the mid-Atlantic may be an orogen of which no portion 

 rises above sealevel. 



MouxTAix Growth 



What forces are involved in the building of a two-sided mountain 

 system and how do they operate? The motive power for folding and 

 thrusting is probably furnished by contraction of the earth. Compression 

 due to contraction is, no doubt, continuous, but in any orogenetie zone 

 periods of deformation alternate with periods of quiet, during which 

 stresses accumulate. But the requirements of gravity must be satisfied 

 in the crust. Tangential movements, with localization of deformation, 

 tend to build up an excess of material in an orogenetie zone, which then 

 subsides under the overload. If subsidence is sufficient, the zone be- 

 comes an area of sedimentation, and is further depressed under the 

 weight of detritus. This marks a geosynclinal or negative stage. As 

 the zone sinks, the geisotherms are depressed; then slowly rise. The 

 increase in temperature and resulting chemical reactions probably do not 

 affect the crust of sediments so much as the deeper portion of the lithos- 

 phere beneath the geosj'nclinal area. Energy arising from the reactions 



