GEOPHYSICAL LABORATORY.* 



Arthur L. Day, Director. 



In the annual report of a year ago some account was given of 

 recent progress toward the attainment of a system of apparatus and 

 of experimental technique for the simultaneous application of high 

 pressure and high temperature in the study of mineral formation. 

 This j^ear it is possible to offer the first results of these studies. 



MINERAL FORMATIONS UNDER PRESSURE. 



It should perhaps be explained that the study of mineral forma- 

 tion under pressure implies somewhat more than may be immedi- 

 ately apparent from this brief statement. The effect of a uniform 

 pressure greater than the prevailing pressure of the atmosphere upon 

 the chemical and chemico-physical relations of solid bodies is very 

 small, so small in comparison with the effect of a change of temper- 

 ature that it may be accounted practically negligible unless the 

 pressure change is very large. The effect of a uniform pressure 

 of 1,000 atmospheres, for example, on the melting-temperature of a 

 metal is (usually) to raise it 5° or 6°, while the greatest effect yet 

 observed with any substance is less than 40°. If the effect on the 

 minerals be of the same order of magnitude, as in all probability it is, 

 then 1,000 atmospheres of pressure, which corresponds roughly to the 

 weight of overlying rock at a depth of 2 miles, would cause a change 

 in the melting-temperature of the minerals of the order of 1 per cent 

 of its value — a factor which might fairly be neglected in considering 

 the conditions of rock formation. 



The foregoing remarks apply to pressure which is uniform, but 

 the question assumes a different aspect if the compression is different 

 in different directions. Such non-uniform compression, which cer- 

 tainly occurs locally at many points of the earth's crust, causes in 

 general a lowering of melting-point and by an amount many times 

 greater than in the simple case of uniform pressure. Such pressure 

 may conceivably lower the melting-temperatures by several hundred 

 degrees and become a factor of great geologic importance. 



If we consider further that a mass of liquid rock (magma) has but 

 small solubility for gases at atmospheric pressure, and bear in mind 

 that substantially all the rocks in nature are found to contain gases 

 and volatile matter in considerable quantity, then we shall see that 

 high uniform pressure has also been influential in the progress of 



*Situated in Washington, D. C. Grant No. 744. $78,000 for investigations and 

 maintenance during 1913. (For previous reports see Year Books Nos. 3-11). 



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