462 FRANK BURSEEVY TAYLOR 
proceeds in a great curve, and if glacial epochs are periodic, as 
they may be, then the variation is expressed by a closed curve, 
either circular or elliptical. 
The manner of retreat under such a combination of forces 
may be represented graphically with approximate accuracy as 
follows: Assuming the oscillation to be superposed upon a 
greater amelioration having a uniform rate, the resulting path 
would be an epicyclic curve. If the recession took the form of 
retreats and halts without readvances the curve would come to a 
point at the moraines as in Fig. 4, A, and the moraines would 
have cross sections, under the simplest conditions of formation, 
like that shown in the figure. With readvances the curve would 
be a iooping epicycle as in Fig. 4, B, (a) and (6), and if the 
readvances covered more than half of each preceding space of 
retreat the loops would overlap as in Fig. 4, B, (c). In both free 
and overlapping loops, the forms of the moraines would be as 
shown in Fig. 1 at A, and in Fig. 4,B,andC. As we have seen, 
this last appears to have been the actual manner of retreat, at 
least in Ohio, Indiana, and Michigan. This is shown in Fig. 
4, B, (c). Ifthe greater climatic variation is truly represented 
by a simple harmonic motion, then the overlapping of the loops 
was most extensive near Cincinnati, and decreased going north- 
ward. This order of retreat is represented approximately by 
Fig. 4,C. The facts seem to favor this method of interpretation 
quite strongly, but it is not necessary to pursue the theoretical 
aspect of the problem further here. It is suggested in this paper 
that precession of the equinoxes may have been the cause of the 
secondary climatic oscillations which produced the moraines of 
recession. But no statement, nor even a definite Opinion, is 
ventured as to the cause of the greater, primary variation of 
climate which brought on the Ice age itself, except that it was 
of an astronomical nature. It may be added, however, that the 
astronomical theory of Croll even as modified and reénforced 
by Ball, is thought not to afford an adequate or satisfactory 
explanation. As to those other various hypotheses which pos- 
tulate purely terrestrial causes, such as the displacement of the 
