THE EARTH'S INTERIOR— ADAMS 257 



But it was a surface repeatedly overwhelmed by outpourings of basaltic lava on a 

 vaster scale than those of later ages, and the crust was recurrently broken up and 

 engulfed in the floods of rising granitic magmas. Here the geologic record 

 begins, but the nature of its beginning points clearly to the existence of a pre- 

 historic eon. At the farther bounds of this unrecorded time, forever hidden 

 from direct observation, lies the origin of the earth. 



But the mind of man will not be baffled. Since he may not see directly he will 

 see by inference. Convergent lines of evidence derived from various fields of 

 knowledge may be followed part way toward this goal, like those rays perceived 

 through the telescope on the full moon near the margin of its visible hemisphere, 

 which converge toward craters on the side of the moon that no eye shall ever see. 



Developments in various branches of science during recent years 

 have enabled us to draw a picture showing, as yet none too clearly, 

 what the interior is like. Any progress that may have been made is 

 due to the joint effort of many investigators, in the laboratory and in 

 the field. The measurements and observations have been inter- 

 preted through the medium of physics, of chemistry, and of mathe- 

 matics. The important part played by these exact sciences in the 

 study of the earth was recognized many years ago. For example, the 

 Advisory Committee on Geophysics of the Carnegie Institution of 

 Washington in 1902 stated in its report: "The phenomena presented by 

 the earth are the historical products of chemical and physical forces." 



SOURCES OF INFORMATION 



We now proceed to discuss briefly the various sources of informa- 

 tion concerning the earth's interior and to list the more important bits 

 of evidence, which may be joined together to give us a notion of the 

 constitution of the earth as a whole. 



The surface of the earth has been thorough!}^ explored, and what Hes 

 just below the surface of the land masses has been carefully and 

 patiently studied by geologists for many years. Thanks to the dissec- 

 tion of the surface by erosion, notably in deep canyons, we are able to 

 learn much about the materials down to a depth of several thousand 

 feet, so that we now have an accurate picture of the rock masses that 

 lie below the superficial layer of soil and sedimentary rocks. Under- 

 neath this thin veneer there is mainly igneous rock, that is, rock that 

 has solidified from molten magma. According to Clarke's estimate, 

 the outer 10 miles of the earth consist of 95.0 percent igneous rock, 

 4.0 percent shale, 0.75 percent sandstone and 0.25 percent limestone. 

 The labors of the geologist have given us a store of information con- 

 cerning the structure and mineral composition of rocks and the 

 interrelations of the various formations. Although there are an over- 

 whelming number of rock types, with respect to composition and 

 texture, curiously enough a predominating amount of the visible 

 igneous rock is either granitic or basaltic. The land surfaces occupy 

 only about one-fourth of the area of the globe. Unfortunately little 



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