230 
facile assumptions of unlimited time. 
He argued that since there is a heat- 
flow through the earth’s crust, meas- 
urable in terms of the downward 
increase of temperature and_ the 
thermal conductivities of rocks, the 
earth could be regarded as a cooling 
globe which must have been progres- 
sively hotter in the past. Beyond the 
dim horizon of the oldest rocks he 
envisaged a “‘beginning”’ correspond- 
ing to the time when the earth was 
molten and newly born from the sun. 
In 1862 he set himself the problem of 
calculating the time that had elapsed 
since the earth’s consolidation. Be- 
cause of uncertainty in much of the 
data, he allowed wide limits to his 
solution, concluding that the observed 
temperature gradients would have 
been notably lower than they are if 
the crust had solidified more than 400 
million years ago, and notably higher 
than if solidification had been com- 
pleted less than 20 million years ago. 
Kelvin’s challenge initiated one of 
the many scientific controversies that 
enlivened Victorian times. Despite 
many protests, however, he finally 
narrowed his limits to 20 and 40 
million years (1897). Archibald Gei- 
kie pointed out in 1899 that the testi- 
mony of the rocks clearly denied 
Kelvin’s thermodynamic inference 
that geological activities must have 
been more vigorous in the past than 
they are today; and that the known 
sequence of sedimentary strata could 
not have accumulated within the 
limits set by Kelvin’s solution of the 
problem. Moreover, James Geikie 
(1900) showed very convincingly that 
the crustal compression set up by even 
100 million years of cooling would be 
confined to an outer shell far too thin 
to accommodate the immense thick- 
nesses of folded rocks involved in the 
Alps and other great mountain ranges. 
Evidently some factor—if not more 
than one—had been _ overlooked, 
though few physicists were then willing 
to admit the possibility of any funda- 
mental mistake. Perry, however, had 
ilready heartened the geologists by 
0inting out that it was allowable to 
ANNUAL REPORT SMITHSONIAN INSTITUTION, 1948 
assume a relatively high thermal con- 
ductivity for the deep interior of the 
globe, in which case the cooling of the 
crust would have been correspondingly 
slowed down. He saw no reason for 
denying the geologists anything up to 
4,000 million years, an estimate of an 
order that has been confirmed by 
Wasiutynski in the course of a recent 
discussion of the earth’s thermal his- 
tory (1946). Nevertheless, Kelvin’s 
great authority compelled a sort of 
compromise, and at the turn of the 
century geologists who claimed more 
than about 100 million years were 
thought to be unduly rash. A few, 
indeed, reluctantly satisfied them- 
selves with a more meager allowance 
of time, but the majority steadfastly 
refused to accept Kelvin’s results as 
final. The real flaw in Kelvin’s as- 
sumptions was disclosed shortly after 
the discovery of radioactivity, when 
Strutt (Lord Rayleigh) detected the 
presence of radium in common rocks 
from all parts of the world. With the 
demonstration that the crustal rocks 
contain radioactive elements and are 
therefore endowed with an unfailing 
source of heat, it became obvious that 
the earth is not living merely on its 
ancestral capital of internal heat, as 
Kelvin had confidently believed, but 
that it has an independent and regular 
source of income of its own. The 
abundance of the radioactive elements 
in the crustal rocks is such that the 
net loss of heat is extremely low; age 
estimates based on the rate of cooling 
are therefore greatly increased. If, 
for example, nine-tenths of the heat- 
flow through the crustal rocks is of 
radioactive origin—and something of 
this order is consistent with the avail- 
able evidence—then the alleged 20—40 
million years of cooling has to be 
multiplied by 100 or more. 
By this time the minds of the older 
geologists were no longer attuned to 
thinking in terms of thousands of 
millions of years and few of them were 
prepared to take advantage of the 
new discoveries. This reluctance was 
largely due to the fact that in 1898 
Joly had resuscitated Halley’s sugges- 
