Geology. 153 



SCIENTIFIC INTELLIGENCE. 



I. 'Geology. 



L The Coral Reef Prohlem and Isostasy ; by G. A. F. 

 MoLENGRAAFF. Kon. Akad. Wettensch. Amsterdam, vol. xix, No. 

 4, 1916. — Professor Molengraaff gives in this paper an interesting 

 and ingenious hypothesis to account for the apparent submerg- 

 ence of oceanic islands, which has given rise to barrier reefs and 

 atolls, without having recourse to those extensive subsidences of 

 the ocean floor, which the Darwin-Dana theory postulates, and 

 which has been held to be an objection to it. 



He gives full weight to the idea of changes in the ocean levels 

 during Pleistocene time by the piling of ice on the land and its 

 subsequent melting, as recently urged by Daly, but following the 

 views of Davis, he believes that to explain the topography of the 

 oceanic islands surrounded by barriers, greater changes of level 

 than could be ascribed to this cause must be accepted. This 

 tends to strengthen the Darwin-Dana theory in its demand for 

 actual subsidence. 



To permit this without recourse to a general subsidence of the 

 ocean bottom, he supposes it to be individual in each case, accord- 

 ing to the following hypothesis. 



He classifies oceanic islands and considers only those rising from 

 abyssal depths as volcanic structures composed chiefly or entirely 

 of basaltic material true oceanic islands. Other islands are to be 

 regarded as those occurring in shelf seas, connected with conti- 

 nental masses, either wholly or partly submerged, and the coral 

 islands found in these shelf seas are to be explained by the "gla- 

 cial-control " hypothesis of Daly. The rocks composing these 

 islands may be of diverse characters. 



The material composing the earth's shell under the oceanic 

 abysses is that of a basaltic substratum, called by Suess the sima, 

 or barysphere, and upon this rests in relative flotation the conti- 

 nental masses of a more siliceous nature and lower specific gravity, 

 or the lithosphere. He notes that isostatic equilibrium is general 

 over the earth, but, since there are mountain masses where anom- 

 alies of gravity exist and isostatic compensation is not complete 

 on the continents, he draws attention to the anomalies of gravity 

 found on true oceanic islands and infers that none of these are 

 isostatically compensated. 



From this he draws the conclusion that, whereas on the conti- 

 nental masses anomalous mountain projections may be able to 

 sustain themselves in virtue of a thicker, stiffer substratum, those 

 rising from the ocean floor rest upon and are rooted in the more 

 plastic basaltic substratum, or sima, and in the long process of 

 time they must gradually sink down and be again welded into it. 

 This gradual sinking down under the influence of gravity is 

 regarded as the cause of the movement of large amount and long 



