FOUNDATIONS OF THE GREAT DEEP 13 



Figure 3 is a photograph of the tube with the thicker "gun" 

 at the top. In 1936 Dr. Piggot thus secured samples at points 

 along a complete traverse across the Atlantic, from Newfound- 

 land to Ireland. These core samples were sliced longitudinally 

 and photographed, with the result shown in Figure 4. Careful 

 study of the cores, inch by inch, has already demonstrated that 

 the deep-sea sampler is henceforth to be regarded as a valuable 

 aid in correlating the physical events on the continents with 

 the evolutionary changes of the open ocean. 



If the layer of deep-sea ooze and mud could be sampled 

 from top to bottom, it would undoubtedly add much to the 

 historical record of a billion years. It is therefore probable that 

 men of science will not rest until they have penetrated as far 

 as possible into the slowly accumulating sediments. Much 

 more difficult must be a sampling of the hard rock beneath that 

 sedimentary veneer. For information about the deeper material 

 geologists are indebted to geophysical methods of investigation. 



Detection with the Seismograph. — The first of two methods 

 to be described depends on a sensitive instrument, the seis- 

 mograph, which, at hundreds of stations distributed on all 

 continents and some islands, registers the arrival of earthquake 

 waves. The timing of arrival is relative to the "instant" of 

 shock at the center of disturbance. Every world-shaking earth- 

 quake sends out different kinds of elastic waves, each with its 

 own time of arrival at any one of the observing stations. From 

 such travel-times valuable data concerning the character of the 

 rocky material all the way to the earth's center have been 

 already secured. 



In order to understand this way of detecting that which 

 must remain forever hidden, we shall make a quick analysis 

 of the seismogram, the writing by a seismograph. 4 For our 

 purpose the essential characteristics of the writing are repre- 

 sented in Figure 5. A seismograph has what may be called a 



