Editorial Comment. 383 
24 million years. But, until we know something more than 
we know at present as to the probable diminution, or still 
conceivably possible augmentation, of thermal conductivity 
with increasing temperature, it would be quite uninteresting 
to publish any closer estimate." 
Lord Kelvin further directs attention to the physical and 
astronomic investigations of Helmholtz, Newcomb, and oth- 
ers, who have shown that the sun's light and heat can have 
been supplied as now probably no longer than a score or a 
very few scores of million years. His article ends with the 
following quotation from the conclusion of King's paper: 
"The concordance of results between the ages of sun and earth 
certainly strengthens the physical case, and tlirows the bur- 
den of proof upon those who hold the vaguely vast age, de- 
rived from sedimentary geolog3^" 
It thus appears, according to the physicists, that the facts 
of geology and of biologic evolution have occupied less time 
than we might suppose indispensible from observations on the 
rate of changes during the recent and historic period. Geol- 
ogists and paleontologists, however, will be reluctant to ac- 
cept so meager an allowance of time, and it seems very desir- 
able that observations from all sources bearing on the problem 
shall be placed on record. Such observations are obtained by 
Mr. G. K. Gilbert from field work for the TJ. S. Geological 
Survey in Colorado, where, as stated by him in the last num- 
ber of the Journal of Geology (vol. iii, pp. 121-127), he sug- 
gests, from alternations of shale and limestone deposits, 
thought to be perhaps referable to the astronomic C3a'les of 
precession of the equinoxes and nutation, that only a part of 
the Cretaceous series m-Aj represent a duration of some 20,- 
000,000 years. This implies one of the highest estimates for 
the age of the earth which has ever been founded on the rate 
of sedimentation. It is quite incompatible with the ph^^sical 
and astronomic estimates before noted, and indeed it far 
transcends the commonlj^ supposed needs of geology and bi- 
ology. These might well be content with Kelvin's 100 million 
years, but cannot well see room for their history if that time 
must be reduced to King's 24 million years. w. u. 
