MECHANICS OF FORMATION OF ARCUATE MOUNTAINS 83 



strata which are dupHcated and reduplicated in the recumbent 

 and "over thrust" folds (Fig. 7, Bh). 



A centrifugal system of active forces implies that a mass relatively 

 rigid and resistant with respect to that within surrounds the arc. 

 Centripetally distributed thrusts, upon the other hand, imply that the 

 area within the arc is relatively the more rigid and less capable of 

 lateral migration. 



Deductions concerning the relative age of Asiatic arcs. — If now 

 we apply this criterion of distribution of thrusts, not to individual 

 arcs alone, but to the Asiatic series as a whole, the centrifugal 

 system of thrusts meets with a no less serious additional difficulty; 

 for it would in this case be necessary to assume that the outer 

 arcs were the first formed and the inner arcs the latest. Upon 

 the other hand, a centripetal arrangement of active forces requires 

 that the reverse should be true. That such is the case appears 

 to be supported by at least four important considerations : namely, 

 (i) the plan of distribution; (2) the position of the more rigid 

 mass; (3) the known geological age of the ranges; and (4) the 

 present locus of seismic and volcanic activity. 



That the outer arcs of the system are, so to speak, laid on or 

 applied to the inner ones is apparent from observation of the 

 outline map (Fig. 2) . More especially is this the case for the smaller 

 arcs attached to the eastern wing of the great Burman Malaysian 

 series and its extension to the north; as it is also for the arcs of 

 Seyestan (Fig. 2, c) and Timor (Fig. 2, g). 



As regards the position of the relatively more rigid area, we 

 know that the Angara platform is an ancient land-mass which 

 has been undisturbed since the Cambrian, whereas outside of the 

 area of the arcs, if we except the remnant of the Gondwana con- 

 tinent in Hindustan and the separated mass of Australia, we 

 encounter only the depressed oceanic basins. 



The inner series of arcs, less clearly marked out morpho- 

 logically as a result of long-continued erosion, we find to be of 

 pre-Cambrian and Permo-Carboniferous age, though accompanied 

 in some instances by later foldings. Farther out we encounter 

 the great Tertiary welts of the Himalaya, Hindu-Kush, and their 

 extensions which have been so recently elevated that erosive 



