FOUNDATIONS OF THE GREAT DEEP II 



other rock types characteristic of the continent project out of 

 the sand and mud of the wide lagoon inside the Reef. 



The cause of the drowning of the hard-rock coastal belt 

 may be obscure, but there is no mystery about the essential 

 origin of the overlying blanket of soft material. All superficial 

 rock of the lands is subject to weathering with disintegration. 

 The resulting solid detritus is washed down by rain and river 

 and ultimately reaches the sea, where waves and currents drag 

 the gravel, sand, and mud outward, far from land. Other such 

 detritus is manufactured by the surf attacking the sea-cliffs, 

 and these other products of erosion, along with countless or- 

 ganic shells and skeletal fragments, are also swept oceanward. 

 The double process has been at work along most of the existing 

 shorelines for millions of years. The result has been the de- 

 velopment of huge embankments of sediment. Proof that these 

 deposits are in general unconsolidated or semi-consolidated 

 will be stated in the third chapter. 



The General Floor 



We come now to the chief subject of the present chapter 

 — the nature of the terrestrial crust beyond the foot of the con- 

 tinental slope. Its investigation has been advanced by the 

 invention of three new tools: improved sampler of the sea 

 bottom; the so-called seismogram; and a specially designed 

 gravimeter, an instrument that measures the force of gravity 

 from point to point on the surface of the sea. 



Bottom Deposits. — Before noting the results of the recent 

 researches, it is worth while to summarize a set of facts won by 

 an older method, using the deep-sea dredge. A large number 

 of oceanographic expeditions, outfitted with the dredge, have 

 disclosed the nature of the first few feet of the bottom sedi- 

 ment. The most comprehensive exploration of the kind was 



