88 WILLIAM H. HOBBS 



The great central remnant of Gondwana land which in Creta- 

 ceous time joined Madagascar to Hindustan, stretching across 

 the central zone of the present Indian Ocean, persisted in large 

 part through the early Tertiary and may account for the main 

 subdivision of the Tertiary arcs to the north of Hindustan as well 

 as the well-defined compressed arc which still exists in part sub- 

 merged to the northward of Madagascar/ 



In later Tertiary time the Malaysian extension of the con- 

 tinent grew to the southeastward and the union of the two great 

 seas to form the present Indian Ocean increased the thrust from 

 the southwest and opposed more strongly that from the Pacific. 

 From the south-southeastward the shielding mass of Australia 

 should reduce the thrust upon the front of this arc and so favor its 

 lateral compression from the oceans on either side. The smaller 

 marginal arcs ranged in series, such as we find in the Philippine 

 archipelago, show in macroseisms continued depression of their 

 fore-deeps, and this indicates that the growth of the arcs may 

 continue at a rapid rate after new arcs (here the Bonin) have begun 

 to form farther out toward the central area of the ocean. 



THE FOLDING PROCESS STUDIED IN THE PLAN — ^EXPERIMENTS 

 WITH CONTRACTING FILMS 



An imitation of arcuate structure may be produced by allowing 

 slightly plastic films on stretched rubber sheets to contract after 

 being locally rigified. This is best accomplished by use of hot 

 Canada balsam spread in thin layer upon a sheet of stretched 

 rubber such as is in use for the manufacture of bellows. An 

 apparatus designed for such experiments is represented in Fig. 8 

 and consists of a strong metal frame within which three brass 

 springs are so related to each other as to outline in the plan a 

 plane equilateral triangle when in unstrained condition. When this 

 triangle is manipulated by simple rods of metal notched upon the 

 lower side, the degree of expansion of the triangle may be varied 

 and maintained for any desired length of time. The springs which 



' This arc is outlined upon Madagascar and in the Farquahar, Amirante, 

 Seychelles, and Coetivy islands, the Saya de Malha Bank, and the Cargados, Mauri- 

 tius, and the Reunion islands (see Fig. 2, p. 78). 



