382 The American Geologist. June, 1895 
The Earth's Age. 
In the first number of JSTature for the present year (vol. 51, 
pp. 224-227, Jan. 3, 1895), Prof. John Perry of London pub- 
lished a criticism of the estimate" reached long ago by Sir 
William Thomson (now Lord Kelvin), from consideration of 
the earth's present crustal temperature, the downward increase 
of its heat, and its probable original temperature when first 
beginning to cool and become encrusted, that the duration of 
geologic time represented by the sedimentar}'^ formations is be- 
tween 20 and 400 million years, and that probably this duration 
must be no more than 100,000,000 years. Professor Perrj'-, 
however, assuming an increased rate of conductivity of rock 
at very high temperatures. Worked out the problem anew, ob- 
taining the result that the earth's cooling to its present tem- 
perature gradient has required more than 9,600,000,000 years. 
This conclusion had been submitted in correspondence to 
Lord Kelvin, whose reply, following Prof. Perry's discussion, 
acknowledges the need of further experiments on the thermal 
conductivity of rocks, but doubts that it would be found to 
vary so as to justify Perry's greatly increased estimate of ge- 
ologic time. 
During the same month (last January) Lord Kelvin con- 
ducted some experiments on highly heated rocks, confirming 
earlier work by Dr, Robert Weber, which together show that 
conductivity does not increase with temperature, which in- 
crease Prof, Perry had assumed. Instead, it is found that 
basalt and marble are practically unchanged in conductivity, 
while rock salt, anhydrite, and quartz show an important de- 
crease. Lord Kelvin therefore, in a recent short article in 
Nature (vol. 51, pp. 438-440, March 7, 1895), still holds to his 
original figures, or is even inclined to reduce them. He quotes 
somewhat fully, and with approval, the paper on this subject 
by Mr, Clarence King in the American Journal of Science for 
Januarj^ 1893, based on the laboratory experiments of Dr, 
(jarl Barus, with diabase under great heat and pressure, for 
the U. S, Geological Survey, This investigation, indicating 
that the earth's age scarcely exceeds the minimum limit of 
Lord Kelvin's earlier estimates, is now regarded by him as 
probably very near tf) the truth, Kelvin says of King's in- 
vestigation : "I am not led to differ much from his estimate of 
