62 CONTRIBUTIONS TO PLEISTOCENE GEOLOGY
Chambers thought that a similar theory would explain certain of
the Scotch terraces, and also those of the Thames Valley, a view
which, at least in its latter application, is untenable.4 But
he advanced at the same time a theory which has received a
considerable amount of approbation, and which is no doubt of
utility in special cases. He regarded the "paired" or "sloped"
terraces such as we have in the Thames Valley (though he did
not apply his argument to this case); as the wings or landward
extensions of ancient deltas raised one after the other at widely
separated intervals and cut through. Thus, supposing for
example that our High Terrace was an ancient delta originally,
after its formation the land was raised and the river cut down
its channel and formed a new delta at a lower level, which, in
turn, was afterwards raised and intersected. After each upward
movement a remnant of the intersected delta would remain on
each side of the valley, forming thus a pair of terraces. This
view was early recognised as forming an explanation for certain
of the structures not completely accounted for by the purely
Huttonian thesis. Many of its later exponents have modified
Chambers' original theory by suggesting alternations of subsi-
dence with the elevatory movements instead of periods of repose,
an innovation which greatly extends the applicability of the
theory in certain cases.
It would, at first sight, appear that the intersected delta
theory affords a ready explanation for the terraces of the Lower
Thames, so far as their continuity and pairing are concerned.
But it explains too little in explaining so much, for, on the other
hand, it fails to account for the local variations in height which
do occur, and which are often not slight. Another objection is
that if the terraces in question really formed during the periods
of their construction extensions of ancient deltas, then we ought
to meet with far more evidence of the proximity of estuarine
conditions than we do.
Having offered these criticisms, we will proceed to state the
opinion which has been forced upon us as a result of our obser-
vations. We have firstly to consider an idea, which, though
doubtless long ascendant in the minds of geologists, first
found its true expression in the writings of Professor
J. W. Powell, viz., that of "Base-levels of Erosion."
4 Ancient Sea-Margins.