DATING THE EARTH'S CRUST—SHAPLEY 149 
could well be cautious and say only that the evidence of the moment 
is fairly strong in favor of the view that the planets, the meteorites, 
the galaxies, the double stars, possibly the clusters of galaxies, and 
certainly the expansion of the galaxies in the observable part of the 
metagalaxy, all date back to T). Now, a time interval of only 300 
million years since T, certainly would not be long enough; and 30,000 
million would be too much. The geometrical mean, 3X 10° years, is 
a convenient number to remember as our current approximation. 
It is not necessary, of course, to go along with Lemaitre’s exploratory 
suggestion and put all the matter of all the galaxies into one original 
cosmic atom, the explosion of which produced the phenomena that we 
now observe. But it is consistent with our various observations to 
accept the former existence of a very highly compressed material uni- 
verse, in which stellar collisions might be numerous, fragmentation 
a common occurrence, and where the prevailing general confusion 
of matter and motions would suit requirements for an initial chaotic 
phase in the development of the solar system. 
Certainly the time T, is a zero point of significance to students of 
galaxies, clusters, meteors, and the origin of the earth. If the cosmic 
violence at that time was sufficiently high-temperatured, it may be that 
the origin of the highly penetrating cosmic radiation should also be 
dated from T,. Some years ago De Sitter suggested that the stars 
themselves may antedate the expansion. And, of course, there might 
have been a long prestellar state of the universe before the shock of 
3X 10° years ago started the currently existing processes. 
It may require active research throughout one or two geological 
periods in order to settle finally the questions we have raised. 
THE FUTURE OF THE METAGALAXY 
In the concluding section of this random discussion of certain 
points in cosmogony, we get completely disconnected from the earth 
and geology. The significant constituents of the metagalaxy in our 
existing cosmogonies are space-time, radiation, the unit galaxies, 
and interstellar dust and gas. We have been content, in the solar sys- 
tem and even throughout our own galaxy, with gravitation as a domi- 
nating force in the moving of stellar bodies. Radiation pressure is 
in general effective only for particles of a diameter comparable with the 
wave length of the radiation. It does not push stars or galaxies 
around. But of late years, as a byproduct of relativistic cosmogony, 
we have come to the concept of a new force—cosmic repulsion. It 
might be called negative gravitation. Its effect can be loosely de- 
scribed as the tendency of material bodies to scatter from each other 
when the mean density of matter in space sinks below a certain ex- 
ceedingly small value. 
It is the cosmic repulsion that is effective in the recession of the 
