128 CARNEGIE INSTITUTION OF WASHINGTON. 



to the sea by erosion, (2) to the considerable differences in the density of the 

 surface rocks themselves above referred to, and to other causes. This theory, 

 singularly well supported by the thousands of measurements of the force of 

 gravity at different points, suggests that all these differences of loading, 

 variable as they are with mountain-building and erosion, great ice-caps 

 which some time covered polar and temperate zones and passed away, are 

 completely equalized through some manner of subsurface adjustment within 

 40 miles or so of the surface. It is as if blocks of heavy wood and cork, 

 soap, and toy balloons were floating together upon water, some projecting 

 high above the surface and some nearly submerged, but the effect of each 

 individual load upon the water completely equalized a foot below the surface. 



This is not an appropriate place to pursue this most interesting subject, 

 interesting especially, perhaps, because of its inaccessibility to direct observa- 

 tion, but even so brief an introduction may serve to indicate how slender and 

 indirect these inferences are, and to explain the eagerness with which every 

 thin ray of light is sought and followed which may conceivably help to pene- 

 trate the dense blackness which hides from our understanding the interior 

 of the earth. Such a ray of light has appeared during the past year in the 

 course of our studies of the compressibilities of the rocks under high pressure. 



Perhaps the most important aspect of pressure as a factor in geologic 

 processes is in its power of holding gases, carbon dioxide, water vapor, etc., 

 in solution in the liquid magma and so influencing both the mineral composi- 

 tion and the manner of crystallization of the rock which forms from it; but 

 the effects of pressure per se are important also, both during the formation 

 process and afterward when exerted upon rocks which differ considerably 

 in their elastic constants. Among the individual effects of such pressures are 

 the production of new and denser forms of minerals under the conditions of 

 high pressure existing even at relatively shallow depths in the earth's crust 

 and the change of density of the rocks and minerals themselves at different 

 depths. These two factors are best investigated by direct measurement of 

 the decrease in volume for a given pressure, i. e., the compressibility. 



The volume change of a mineral or rock under pressure is difficult to 

 measure because it is so small; a pressure of one atmosphere decreases the 

 volume of an ordinary rock by only one or two millionths of the original 

 volume. With many rocks porosity introduces a further complication. 

 Heretofore the compressibility of rocks has been determined only by an 

 indirect method, but we have been able to measure the volume change 

 directly under hydrostatic pressure. This is made possible by working at 

 very high pressures and by inclosing the rock specimens (which are sur- 

 rounded by liquid) in a thin jacket of soft metal. The technic of high- 

 pressure experimentation no longer encounters the formidable difficulties 

 which once seemed nearly insurmountable; and, with hydrostatic pressure, 

 measurements can be obtained at much higher pressures than by the indirect 

 method. Furthermore, soft rocks of imperfect elasticity are just as amenable 

 to measurement as the harder and more elastic rocks. Altogether, we have 

 now measured the compressibility of 40 materials up to pressures equivalent 

 to a depth within the earth of 25 miles. 



From compressibility we are able to pass to another elastic property, the 

 rigidity, and to assign a value to the rigidity of any rock whose mineral 

 content is known. The rigidity of a granite is the smallest of the typical 



