236 CHALLENGER 



Soon the ship moved more rapidly to Hong-Kong, Singapore 

 and into the Indian Ocean through the Sunda Straits. Here good 

 weather allowed a feast of seismic experiments, and scientific 

 morale onboard rose accordingly. By now a considerable num- 

 ber of seismic experiments had been made in the Atlantic, Pacific 

 and Indian Oceans, and still there were some more to be made 

 in the Mediterranean. Tom and John had foimd much of interest, 

 but possibly the fact that the earth's structure beneath the true 

 oceans is radically different from that beneath the continents is 

 the most striking result of seismic experiments at sea. In Challenger 

 they had measured with refraction shooting the thickness of the 

 sediment carpet on the ocean floor composed of the calcerous 

 or silicious skeletons of countless millions of planktonic animals 

 and plants finding a great difference in thickness from one part 

 of the ocean to another and also between oceans. These sediments 

 have a speed of sound through them little faster than in sea water. 

 In the Atlantic various thicknesses were measured from 3500 feet 

 to practically nothing, whereas a carpet depth of about i ^00 feet 

 was more usual in the Pacific. 



The rate of accumulation of this sediment on the sea-bed has 

 been measured by inspecting core samples and identifying volcanic 

 ash strata blown over the surface of the sea from two separate 

 eruptions in historic times, the distance between these two dark 

 layers in the core being measured as one would measure the dis- 

 tance between two layers in a cream-filled cake. A little under 

 half an inch in 1000 years seems to be a popular estimate thus 

 obtained, but it seems doubtful if there are many places in the 

 oceans where the carpet is laid down from sediment falling only 

 from directly above. Sub-surface and bottom currents flowing 

 upon as yet undiscovered courses probably carry sediment, like 

 snow on the winds, to form drifts upon the ocean floor. 



Beneath the sediments in true oceanic areas the first layer of 

 rock would appear to be of great hardness indicated by a speed 

 of sound of about 22,000 feet per second, which continues down- 

 wards until the earth's true crust is reached about 36,000 feet 

 below the surface of the sea. The meeting of these two is known 

 to scientists as a well-defined discontinuity, which under the con- 

 tinents is not reached until nearly 100,000 feet has been pene- 

 trated. This startling difference in structure between continental 



