Progress of Geology y 1 891 -1 91 5 
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matron, or a gibbering old woman. The margin of safety in these 
guesses is about one billion years. 
In 1862 Lord Kelvin, studying the thermal conductivity of the 
sphere, decided that the earth is at least 20 million and not over 400 
million years old. After an interval of 35 years he amended these 
figures somewhat and stated, in concurrence with Clarence King's 
assertion, that about 24 million years ago the earth was a molten 
mass. 
Sir George Darwin in 1886 had urged the wisdom of considering 
“ Theories which appear to demand longer periods of time than those 
which now appear allowable." Ten years ago he suggested again that 
the physicists may be in error in computing the age of the earth, 
and said: ‘'The scale of geological rime remains in great measure 
unknown." 
From the thickness of sediments, and the rate at which rivers are 
making deposits, Charles D. Walcott in 1893 reckoned the lapse of 
time since the beginning of the Archean to be 90 million years. 
Professor John Joly in 1899, from computations of the quantity 
of sodium in the oceans and their annual accession of sodium, stated 
the probable age of the earth to be from 90 to 100 million years. 
Students of biology have preferred a great length of time for the 
complex results of evolution. The estimates made by physicists, 
chemists, and some geologists appear inadequate to them. 
Since the discovery of radium, and a more general investigation 
of radio-active minerals and derivatives from radium, the evolu- 
tionists have taken hope. No line of investigation has been so 
profligate with time as that concerning radio-activity. John Allen 
Harker, the British physicist, says, “a study of the various radio- 
active elements contained in minerals and rocks has shown that it is 
possible, in certain favorable cases, to calculate directly their ages in 
years." Thus calculated, the Archean rocks are from one billion to 
one billion, six hundred million years old. 
Over a hundred years ago, Hutton, speaking as do the poets and 
the prophets in science, asserted that geologic time had “no vestige of 
a beginning, no prospect of an end." 
Theories of Earth-Origin. Twenty-five years ago one seldom 
heard any question raised about the satisfactoriness of the nebular 
h 3 rpothesis. More recently certain variations in this hypothesis have 
been proposed, but the fundamentals of the Laplacian theory have 
place in all these restatements. Necessarily, modifications should 
