438 FRANK BURSLEY TAYLOR 
retreat has not been associated by Professor Chamberlin with the 
causes here suggested. According to his view the drift of each 
main ice invasion was laid down in imbricate fashion; that is, 
there wasa continual oscillation with moderate readvances as the 
general retreat progressed, so that the drift was laid down in 
successive overlapping sheets somewhat like the weatherboards 
on a frame house, or the shingle rows on a roof. In Geikie’s 
Great Ice Age, under ‘‘The Imbrication of the Drift Series,’ 
beginning on page 736, his views are given as follows: 
The drift deposits of the great plain region of North America may be 
looked upon as a series of sheets overlapping each other in imbricate fashion; 
the outermost disappearing beneath the next inner, and this, in turn, dipping 
beneath the succeeding, and so on. The outer uncovered zone of each sheet 
retains its original form, except as modified by superficial agencies, but the 
inner buried zone was much modified by the over-riding ice during the later 
advances. Ina general view of the drift, itis important to grasp clearly this 
conception of the overlapping of the sheets, and to distinguish this imbricate 
structure from the simple stratigraphical superposition of marine sediments 
on the one hand, and of simple morainic corrugations following each other 
in concentric recessional lines on the other. It is, furthermore, important to 
observe that this is only a superficial conception of the drift series. Theoret- 
ically, there are at least two of these imbricate series for every period of gla- 
ciation, and the order of imbrication takes on opposite phases. During the 
first part of the glaciation, when the ice on the whole was extending, though 
by alternate advances and retreats, the later were generally greater than the 
earlier advances. During the succeeding stage, however, when the ice was, 
on the whole, retiring (though by oscillations) the later advances generally 
fell short of the earlier. Inthe case of the lower or older series of glacial 
accumulations, therefore, the later deposits generally reach farther south than 
the earlier ones, whereas, during the recessional stages of glaciation, the 
earlier sheets extend farther south than the later. These two imbricate series 
of sheets of contrasted order represent the two great halves of a period of 
glaciation. If there were two or more entirely distinct periods of glaciation 
theoretically the double imbricate series repeated itself accordingly. .. . 
There is one other class of facts that may ultimately be 
added to the proof of readvances in the oscillations. Bowlder 
belts, at least in certain situations, are believed to indicate read- 
vances. Respecting the source of the bowlders themselves it 
seems safe to say that ninety-nine out of every one hundred in 
