DROWNED ANCIENT ISLANDS — HESS 291 



intense volcanic and seismic activity.'* On the eastern margin are 

 found the cordilleras of the North and South American west coasts and 

 on the south littlQ-known Antarctica. The volcanic rocks of the 

 islands of the Pacific Basin are dominantly basaltic, whereas those 

 related to the island arcs and their uplifted cordillera equivalents are 

 dominantly andesitic. The area of arcs and cordilleras bordering 

 the Basin is tectonically the most active and unstable area of the 

 earth's crust today. The Pacific Basin itself seems to be tectonically 

 a most stable area and possibly has been throughout geologic time.^ 

 One encounters no evidence of folding anywhere over its broad ex- 

 panse. Though fault scarps can be found, their rarity bespeaks great 

 stability. Seismic activity in the Pacific Basin is almost nil. 



The writer favors Buddington's (1943) concept of the nature of the 

 earth's crust and considers that the suboceanic crust probably consists 

 of horizontally layered rocks including such types as norite, gabbro, 

 anorthosite, pyroxenite, peridotite, dunite, and probably some eclogitic 

 facies. These are relatively strong rocks — stronger than the granitic 

 to quartz dioritic rocks which presumably make up the "granitic" 

 layer of continents. The writer believes the oceanic crust is very 

 strong though this opinion is at variance with existing textbooks and 

 much of the current literature. However, Jeffreys (1929), Daly 

 (1940), and Longwell (1945) all favor a strong oceanic crust. The 

 only bases for judging its strength are its behavior and the strength 

 of the rocks of which it is thought to be composed. Both of these 

 indicate strength. The reason it has been generally considered to be 

 weak, appears to be related to calling it the exposed sima or the 

 basaltic substratum and consciously or unconsciously bringing in 

 Daly's theory of a weak glassy basaltic substratum. But Daly post- 

 ulated a strong crust and weak substratum at considerable depth. 

 Those favoring the hypothesis of continental drift assumed a very 

 weak basaltic crust below th^ oceans without, so far as the writer 

 is aware, presenting evidence other than the hypothesis of drift to 

 substantiate assumption. 



* There Is penernl agreement as to the position of the "andesite line" along the western 

 margin of the Pacific Basin except for the area of the Carolina Islands. Some place these 

 Insifle and some outside of the "andesite line." The writer tentatively includes most of the 

 Carolines in the Pacific Basin and traces the "andesite line" down their western margin 

 Including Ulithi, Tap, Ngulu, and the Palaiis hehind — on tlie west side of — the "andesite 

 line." This is essentially the same as the line drawn liy Hobbs (1944). 



* Having obtained considerable first-hand Information in the Pacific diiring the past 

 few years the writer must now revise the views expressed in Betz and Hess (1942). The 

 tentative trend lines shown on the chart should be considerably reduced in number by 

 eliminating practically all of northeasterly trends. Further development of the bottom 

 topography shows that they do not exist. The hypothesis that certain linear groups of 

 islands and shoals, particularly tlie Hawaiian group, lie along a major earth fracture which 

 may be a strilce-slip fault is retained. The rclationsliip on a small scale of the volcanic 

 activity to fractures has been demonstrated by Stearns and MacDonald in Samoa and 

 Hawaii. The trends of these fractures are approximately parallel to the elongation of 

 Samoa and the elongation of the Hawaiian chain. 



