458 FRANK BURSLEY TAYLOR 
greater, slower climatic change. After the grand climax of 
advance, when the ice of this epoch reached its farthest point 
south, the long retreat began and the oscillations that made the 
moraines of recession went on round after round during the slow 
progress or the greater change. This is a truth that stands out 
clearly on the face of the larger facts and is entirely independ- 
ent of all theories as to the cause of the oscillations or of the 
greater change itself. Here again the moraines of recession, by 
their arrangement, and by the regularity of their intervals, help 
us to a partial insight into the nature of this greater change — 
the real cause of the Ice age. Not that they show us the whole 
cause fully and clearly, for they donot. But they show us enough 
of its real nature to enable us to eliminate several hypotheses 
that have been suggested, and so to narrow the range of discus- 
sion. The cause of the greater change must have been of an 
astronomical nature, and there is apparently no alternative. 
In order to see the full import of the facts we must see just 
how the oscillation is related to the greater change. If the 
oscillations were regular (either with equal time intervals, or 
with time intervals that varied progressively at a uniform rate) 
and the greater change also regular (uniform or varying at a 
uniform rate) then the moraine series would tend to be regular, 
but if either one or both of the changes were irregular, then 
the moraine series would be irregular. To getaregular moraine 
series out of a combination in which either factor was irregular 
would be accidental, and an assumption that such a cause has 
produced the moraine series would be gratuitous and without 
reasonable foundation. 
A careful analysis of the effect of topography in causing 
irregularities in the moraine series seems to show that the inter- 
morainic intervals are not uniform, but increase from south to 
north. By reference to. the map it will be seen that from the 
first moraine back to the ninth, or Fort Wayne moraine, the 
intervals are shorter than from Fort Wayne northward. The 
effect of topography can be best understood by considering the 
glacier in its advancing phase. 
