author's views and conclusions 249 



for the arc. In this respect the writer's view differs materially from that 

 of Brouwer, who sees in the straits which separate the islands of an arc 

 the evidence of late tensional movements which he supposes to result 

 from a forward migration of the arc by a push from behind.-^ The evi- 

 dence that the arcs of today rise first at special points appears to be con- 

 firmed by those older arcs which skirt the eastern flanks of the Rocky 

 Mountains.^ 



j^o sooner does an anticline rise to form an arc than an elongated and 

 roughly parallel trench — fore-trench — ^begins to develop along its front, 

 and the crucial test to determine whether the arcuate arrangement of a 

 group of islands may not be fortuitous, and hence without tectonic sig- 

 nificance, is exactly the presence or absence of such a fore-trench. Hence 

 the great importance of carrying out an elaborate series of soundings in 

 the neighborhood of all growing mountain ranges of the oceanic areas. 

 There is in the development of the fore-trench opposite a rising arc an 

 apparent indication that the void which should tend to develop beneath 

 a rising arch in the strata is met by a lateral migration of subsurface 

 material from in front of the rising anticline — an isostatic adjustment 

 which follows as a consequence of mountain erection, but is in no sense 

 an initiator of it. The anticline and syncline from this stage appear to 

 develop together. In those later stages where arcs have taken on a sharp 

 curvature the trench appears at the back instead of the front of the arc, 

 notably in the examples of the Moluccan and Windward arcs. 



Within coral seas where atolls are to be found, the earliest evidence of 

 arc formation may be the elevation of such atolls, and these may be de- 

 tected today in very many cases by their deposits of phosphate, such 

 islands being far more numerous than is generally supposed.'^ 



In the study of the young island arcs, examination of each island is 

 essential in order to determine its individual character and its relation 

 to the arc as a whole. The study of the Marianne arc, which is in an ex- 

 tremely youthful stage, has brought out the fact that this arc is made up 

 of three distinct zones from front to back, and that it betrays a no less 

 marked differentiation when examinted in a direction from end to end. 

 At its southern end it is indicated merely by a raised atoll, that of Feys. 



^ H. A. Brouwer : The horizontal movement of geanticlines and the fractures near 

 their surface. .Tour. Geol.. vol. 29, 1921. pp. 500-577. Fractures and faults near the 

 surface of moving geanticlines. II. Abnormal strikes near the bending-points of the 

 Iiorizontal projection of the geanticlinal axis. Troc. Roy. Acad. Sci. Amsterdam, vol. 

 25, 1922, pp. 229-334. 



« Earth evolution and its facial expression. Macmillan, 1921. pp. 139-142. 



'^ W. H. Hobbs : Les guirlandes insulaires du Pacifique et la formation des montagnes. 

 Conference faite a la Sorbonne le 29 avril, 1922. Ann. de Geog., vol. 31. 1922, pp. 

 485-495 



