cases the flattening was then impressed. Now, inasmuch as the tides, lunar and solar in the case of the Earth, solar practically alone in the case of Mars, have been slowing up the planet's rotation ever since this refrigeration happened, but as their respective rates of rotation still agree substantially with what a fluid condition demands, it is evident that in the case of neither planet could the cooling have begun so very long ago, but that it began longer ago for Mars than for the Earth.

In so far, then, we trace a certain similarity of development in the early chaotic stage of evolution of the two planets, a stage pre-natal to their career as worlds.

From these basic facts of size and shape we will now go on to more latter-day detail.