ORIGIN OF THE EARTH—PAGE 163 
THE RECORD IN THE ROCKS 
In geology it is assumed that we can explain past developments on 
the basis of processes taking place today, and this assumption has been 
remarkably successful in tracing geological history to form a consistent 
pattern. The surface features of the earth can be explained as the 
expected result of erosion, of glacier action, of volcanism, and of move- 
ments of the crust itself, all of which are observed in action now. This 
reasoning might be expected to lead, step by step, to the origin of the 
earth. 
The sequence of events in earth history is best summarized by the 
geologic column, a schematic pue of all the rock strata which have been 
classified, in the order of their formation. After fitting together rocks 
from all over the world, there are left only four major gaps in the 
record, when erosion in practically all parts of the earth now above 
sea level must have eliminated the rock deposits of millions of years. 
With these four exceptions, the geologic column, fitted together from 
the results of a century of world-wide geologic prospecting, gives al- 
most as complete and consistent a picture of earth history as if the 
entries had been meade in a diary. It lacks only the number of years 
intervening between the various geologic eras. 
The dates were supplied when the absolute ages of rocks were esti- 
mated from their radioactivity, first in 1905 by Boltwood, an American 
physicist. He measured the relative amounts of lead and helium in 
uranium deposits. The uranium ore crystallized when the molten 
magma solidified, and the radioactive uranium has since been dis- 
integrating at a constant but very slow rate to form lead and helium 
which, in favorable cases, have both remained sealed in the igneous 
rock with the uranium. The process of radioactive decay has been 
thoroughly studied in the laboratory by many physicists, including the 
Curies and Rutherford (who suggested Boltwood’s research), and the 
rate of disintegration accurately measured. 
Dating various igneous rocks in the geologic column showed first 
how very long was the record; the oldest igneous rocks yet dated 
crystallized about 3 billion years ago. Moreover, there are even older 
sedimentary rocks through which the molten magma had pushed to 
form these oldest known igneous rocks; hence the earth must have had 
surface conditions about 3 billion years ago not radically different 
from those today. There must have been water and an atmosphere 
operating to erode rocks and form sand and mud beds. Fossils in 
somewhat younger rocks indicate that early forms of life existed at 
least 1 billion years ago when conditions must have been very like 
» those today. 
