MECHANICS OF FORMATION OF ARCUATE MOUNTAINS 87 



should be diminished by any folding upon the opposite end of the 

 arc, by reduction in volume of the strata, by any closing-up of 

 joint spaces, etc. 



In this connection it should not be overlooked that when the 

 stage of sUding has been reached in the process of folding, the sur- 

 faces of dislocation in nearly horizontal attitude so facihtate under- 

 riding of strata that this may follow contraction without the 

 necessity of infall of the depressed areas. 



Form of the arcs and their distribution an expression of the 

 space relations of continental pedestal and ocean floor. — If we are to 

 assume the ocean basins to be the great loci of dispersal of com- 

 pressive tangential stresses within the earth's shell, as brought 

 out in the last section, we should examine each one of the Asiatic 

 series of arcs to note whether it correctly expresses what is known 

 of the distribution of ocean and continent and of deepening or 

 shallowing conditions in the former at the time of arc evolution. 

 In pursuing this inquiry we note that the Permo-Carboniferous 

 arcs to the southward of the Angara coign should have a general 

 east-and-west trend in order to correspond to thrust from the 

 Tethys Sea which at the time of their formation stretched in that 

 direction over what is now southern Asia and separated Angara 

 land from the great Gondwana continent to the south. To the 

 eastward of the ancient coign the general trend of the arcs should 

 not, however, differ greatly from that of the later arcs in the same 

 vicinity or of the shore line of today. 



The extension of the Angara continent southward over the 

 Paleozoic Tethys, and the formation of the Indian Ocean by the 

 breaking-up and partial sinking of the Gondwana continent pre- 

 vious to the folding of the early Tertiary period, is likewise in 

 conformity with the greater accentuation of the curvature of the 

 Tertiary welts across southern Asia. The growing peninsula 

 which in Tertiary time connected the Malaysian archipelago to 

 Asia was partially protected from southerly thrust by the mass 

 of AustraHa, while being pinched between the eastern sea in the 

 Indian Ocean and the vast Pacific. We may thus perhaps account 

 for the elongation of the Burman-Malay arcs and their note- 

 worthy lack of symmetry as well. 



