author's views and conclusions 247 



On the other hand, if the system of thrusts has gone out from the oceanic 

 areas and been directed against the margins of the continent, the coign 

 of Australia must be regarded as a protective shield which has warded off 

 any thrust from the southeast, while the thrusts on the Malayan region 

 from its opposite sides (northeast and southwest) should have resulted 

 in a much greater compression of these arcs than is elsewhere to be found. 

 In a broad way, the map and section B represent the facts so far as they 

 are known. 



An obvious cause of the landward thrust from the sea, which is here 

 invoked as the cause of formation of mountain arcs, is the still continuing 

 subsidence of vast areas in the Pacific and Indian oceans, to wiiich the 

 formation of the groups of atolls bear testimony. 



The marked zone of seismicity and the line of active volcanic vents 

 which alike follow the course of the festoons of islands off the coast of the 

 Asiatic continent seem clearly to reveal them as the very youngest of the 

 Asiatic arcs and those which are in the process of erection today on the 

 floor of the sea with their summits just emerging from the waves. Be- 

 lieving that, in geology as in zoology, it is in the embryo that the life 

 history is often most clearly revealed, tlie writer during the summer of 

 1921 carried out a reconnaissance of these youngest arcs, in connection 

 with which he visited islands of the Bonin, Sulphur, Marianne, Caroline, 

 Pelew, and other groups with a view especially to find evidence of con- 

 tinuing mountain growth and to check this by observing the stage of the 

 erosional process, as well as the nature and comparison of the reef struc- 

 tures on the opposite sides of the arc. These latter are of particular value 

 in revealing the structure of an anticline, and in practically all earlier 

 stages of arc formation proceeding within the tropical seas, wherever 

 conditions suitable for reef-making have existed, elevated reef-caps ap- 

 pear as terraces on the convex outer side of the arc and barrier reefs 

 indicating subsidence on the opposite or concave side. 



In a later stage of arc formation alternating vertical movements of 

 great amplitude develop along the medial portion of the convex side of 

 the arc, so that here also barrier reefs develop. These central zones off 

 the front of an arc in a late stage of its development are regions of special 

 unrest of the sea-floor and in this respect surpass anything whicli is else- 

 where known. Here are the special loci of seaquakes which 'have been 

 such a menace to the navigator, while at the same time the arcs are the 

 vigias or sign-board warning of the danger. 



The erection of an evolving anticline does not appear to take place at 

 a uniform rate throughout, but there first arise a series of domes in 

 arcuate arrangement which only later unite to form a uniform crestline 



XVII — Bull. Geol. Soc. Am., Vol. 34, 1922 



