234 W. H. HOBBS ORIGIN OF OCEAN BASINS 



and valuable data for a solution of the question of age of the present 

 ocean depressions. Many of the arguments once urged in support of the 

 antiquity of the present basins, interpreted in the clearer light of today, 

 inveigh against it. These arguments have perhaps nowhere been better 

 stated than in the anniversary address of the late Professor W. T. Blan- 

 ford before the Geological Society of London.* 



The arguments for the permanency of the present ocean basins are : 



I. The supposed absence of continental rock types from oceanic islands, 

 thus allowing it to be inferred that the ocean bottom, like the islands 

 themselves, is composed of heavier volcanic rock. 



II. The abnormal gravity values obtained in pendulum observations 

 made within the infra-continental zones and supposed to be influenced 

 by a heavier stratum of rock which occupies the ocean floor. 



III. The general absence of typical deep-sea deposits from the conti- 

 nental areas. 



IV. The supposed restriction of related faunas and floras to the same 

 continent. 



Supposed Absence op continental Eocks prom oceanic Islands 



As regards the first mentioned argument, more non-volcanic oceanic 

 islands have been explored since the doctrine was first brought forward, 

 and there have been discovered in the lavas of oceanic islands inclosures 

 of rocks belonging to the continental types; which discoveries have re- 

 moved much of the force of the argument. 



Most of these oceanic islands project above the surface of the sea as the 

 upper parts of the highest volcanic accumulations. Examination of vol- 

 canic districts upon the continents speaks for a basement of non-volcanic 

 material. Should the cinders be largely submerged the high volcanic 

 peaks would be the only parts exposed above the sea. 



Abnormal Gravity w^ithin inpra-continental Belts 



The writer has elsewhere f drawn attention to the many cases in which 

 derangement of the gravity constants appears to be associated with earth- 

 quake zones and dislocation belts and the way in which this phenomenon 

 appears to be related to magnetic disturbances. If it be true that the 

 crust adjusts itself to the secular cooling in compartments bounded by 

 faults (which occasionally are laid bare at the surface), and if, further, 

 it be true that the dense core of the lithosphere (specific gravity, 5.6) is 



* Quart. Jour. Geol. Soc, vol. 46, 1890, pp. 60-110. 



t Gerland's Beitrage zur Geophysik, vol. 8, 1006, heft 2, pp. 271-277. 



