tricity of the terrestrial orbit are reflected on the geological series of 
strata, and are the key to the calculation of the duration of epochs. 
The precession causes the winter and summer to be alternately 
longer and shorter. In the semiperiod when winter is longer than 
summer, the distinction between inland and coast climate becomes 
more prominent. The currents of the atmosphere become stronger, 
and in consequence of that, the ocean currents increase in 
strength, and that again reacts upon the climate. The periodical 
change of the climate produced by the precession is not great, but it 
is sufficient to imprint itself in the alternation of beds, and in the 
formation of beach-lines, terraces, series of moraines, etc. To each 
period of precession corresponds one alternation of strata. 
The eccentricity of the Earth's orbit is periodically changeable. 
Its mean value rises and falls for a period of about 1)4 millions 
of years, with i6 oscillations. Such a rise and fall I term a cycle, 
and each cycle is, in the calculated curve, composed of i6 arcs. 
The tidal wave, which is the most powerful agent in altering the 
sidereal day and in lengthening it, rises and falls in some measure with 
the eccentricity. It so exceeds the other forces that act in altering 
the length of the day, that the day steadily becomes lengthened, on 
the average, more quickly in the middle of the cycles, when the 
mean value of the eccentricity is greatest, and more slowly at the 
limit between them, when the eccentricity is the least; and in respect 
of the respective arcs with increasing speed during falling eccen- 
The interior of the globe is plastic, owing to great pressure. 
The surface or " crust " opposes the greatest resistance to change of 
form. But according as the sidereal day becomes lengthened, and 
the equatorial regions of the earth increase in weight; a steadily 
increasing strain acts outward towards higher latitudes, and the 
in mind that forces too slight to produce a sudden change in a 
solid body, may still produce a change of form when they act 
through long periods. Therefore the lengthening of the sidereal day 
acts not only on the seas, but also on the form of the solid globe. 
The earth approaches steadily more and more to the spheriform, 
but the solid crust is more sluggish in its movement than the seas, 
which immediately accommodate themselves to the altered time of 
rotation. As the motive force of these movements of seas and 
solid earth is periodically changeable, according to the eccentricity 
of the earth's orbit; these movements take place also, periodically 
quicker and slower. And as the seas always accommodate them- 
selves to the forces before the dry land does, it is likely that the 
