The nature of the earth at its surface is apparent to us, and we may make direct physical and geological observations of the conditions in the surface layers. Mines and bores have penetrated to a certain depth, and natural cracks and upheavals show us something of the nature of the disturbed crust: but the greatest depth to which a test bore has been taken is eleven thousand feet. Whilst that seems a very great depth, we must not forget that it has only stabbed to one-four-hundredth of the total distance to the centre of the earth, and that only at one place. For further information geophysicists are dependent on the manner of the formation of the world and the observed nature of its parent, or on examinations of the records of naturally created and transmitted earthquake shocks. By a section of applied geophysics known as geophysical prospecting he may acquire a considerable amount of information as to the physical nature and disposition of materials in the earth's crust down to a depth of a few thousand feet. A great amount of our modern theory as to the nature of the world depends, then, on the theory of the method of formation.
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