344 



THE AGE OF THE EARTH. 



seen to be of the same initial excess as tbe Kelvin .'>,000'^ C line. In 

 spite of its long cooling even after 237x10'* years it is still very far 

 from solidity. Of tbe original Huid conche of 22G miles, only about CO 

 miles bas been congealed into crust, 1(!0 miles remaining fused. 



Gradient /<, initial excess 2,500'^ C, and a 100x10'' years refrigera- 

 tion bas an original fluid coucbe of 120 miles witb a ]>resent crust of 

 50 miles and an existing residual coucbe of fusion of 04 miles, a case 

 also inadmissible from tbe i)oint of view of instability. 



Gradient V, initial excess l^lGiP C. (platinum melting point ), and 

 40x10'' years of cooling, bad originally a 5.3-niiIe surface coucbe of 

 fusion wbicb bas long since i)assed into solidity. Tbe following table 

 sums u}) tbe condition of all tbe gradients as to initial excess, initial 

 deptb of surface fusion, time of cooling, tbickness of crust congealed, 

 and present residual coucbe of fusion: 



Table (\. — Liquid solirl couditionsfor diabase earth. 

 A. — Gradients having the surface rate of 50'G to 1° F. 



B.— Gradients having the surface rate of 7;'' feet to 1° F. 



Comparison of gradients of equal initial excess and successively 

 longer periods of secular cooling sbows tbe ratio of tbeir retreat from 

 rigbtto left across tbe cbart or from lower to bigber values of deptb 

 and time. 



Witb eacb augmentation of age tbe initial tangent defining tbe sur- 

 face rate is seen to bave declined furtber and furtber from tbe original 

 rate co , coinciding witb and passing first tbe maximum, tben tbemini- 

 nunu rate tbence declining into tbe region of inadmissible rates. 



Tbe probable conditions of tbe true gradient are as to initial excess 

 and age sucb as fall below tbe diabase line into solidity and emerge at 

 the surface with a rate wbicb bas not declined below tbe mean (B A) 

 rate of 04 feet to 1*^ F. From tbe point of view of solidity no gradient 

 of initial excess above 2,000° 0. is admissible; tbat of 2,500° C, even 

 after 100 x lO'' years cooling, still sbows a dee)) sbell of fusion (04 miles 



