SEDIMENT CARPET 63 



tion measurements to great ocean depths. The results 

 he has obtained, largely from the northwest Atlantic 

 Ocean, as well as those more recently found by the 

 English investigator M. N. Hill working from one of the 

 weather ships in the northeast Atlantic, are truly re- 

 markable and must be briefly summarized. 



What lies beneath the sediment carpet, the rocky 

 basement of the ocean floor? This problem has 

 for generations puzzled geologists and geophysicists. 

 Seventy years ago Eduard Suess, the great Austrian 

 geologist, put forward the hypothesis that the ocean 

 floor is built of a material different from that of con- 

 tinental rocks and representative of a deeper shell of the 

 earth's crust. This material he called "sima." It was 

 assumed to have a more basic chemical composition 

 than the continental surface rocks, which are largely 

 granitic and have aluminum silicate as their dominating 

 component, whereas in the lower, less acid rocks 

 silicates of magnesium are dominant. Hence the 

 abbreviations "sial" and "sima," which have been 

 adopted by most authors dealing with the subject.* 

 This differentiation between the two upper layers of the 

 crust was further elaborated by another Austrian, Al- 

 fred Wegener, who assumed the continents to be blocks 

 largely made up of sial floating on a substratum of sima, 

 which because of a higher specific weight and a greater 



* Quite recently Benno Gutenberg has adopted a somewhat 

 modified nomenclature. See p. 65. 



