336 THE AGE OF THE EARTH. 



A recent investigation of tlie rock diabase in its relations to lieat 

 and i)ressure etlers tlie formerly lacking means of testing the admissi- 

 bility of many cases of earth-temperature distribution from the point 

 of view of solidity. Ten years ago in a laboratory established by me 

 in connection with the U, S. Geological Survey, Dr. Carl Barns began 

 a series of experimental researches tending toward the solution of some 

 of tlie Tuiknown bnt important points of geological physics. It has 

 been my i)rivilege to indicate the direction of much of the inquiry. 

 The understanding between us maintained his entire independence in 

 the mode and ])rosecution of the investigations, and secured for him the 

 fnllest lesponsibility and credit for the purely i)hysical results, many 

 of which have at intervals appeared in this aud other Journals. For 

 myself was reserved the privilege of making geological api>lications of 

 tlie laboratory results. One of the most important of these is Dr. 

 Barus's lately completed determination of the latent heat of fusion, 

 specitic heats melted and solid, and volume expansion between the 

 solid and melted state, of the rock diabase.* To him I am also very 

 generally indebted for aid in considering the present problem. 



Diabase was chosen l)y me as fairly illnstrative of the probable den- 

 sity and composition of the surface O-O.") or ()-04 of the earth's radius. 

 For Laplace's law of distribution, density at the surface is taken ar 

 2 7;") and down one-tenth of radius at 3-8S, yielding- a mean density of 

 the whole tenth of '^-oo and for the uppcir ti\ e-hundredths of about 3. 

 For the whole tenth a rock like the extremely heavy basalt of Baren- 

 steint (sp. gr. 3-;)5) would api)roach closely a fair mean expression of 

 density. Typical hornblende andesite comes closest to the average 

 density at the suriace, but diabase (sp. gr. 2*8 to 3), nearly enough fills 

 the conditions of the shell which this study seeks to investigate. The 

 particular diabase under <*xaminatiou came from Jersey City, and was 

 taken from the immediate vicinity of the Pennsylvania llailroad cut. 



The ibllowing analysis is by (t. W. Hawes:| 



Silica - 53-13 



Alumina 13 74 



Ferrous oxide 9 10 < 



Ferric oxide 1 08 



Manganous oxide 43 



Lime 9 47 



Magnesia 8 08 



Soda 2-30 



Potasli 1 •03 



Ignition -90 



90. 70 



Astronomical and geodetic requirements make necessary that density 

 should proceed downward iu shells of successively greater value, but 

 the suriace density is 2-75 and the mean density of the whole earth is 



* Avi. Jour. ScL, December, 1891, aiul .January, 1892. 

 t J. Koth, Gesteins-Aval!)xcii, ISGl, p. 46. 

 \ Am, Jour. Set. (Ill), vol. ix. 



