GEOLOGY: IV. M. DAVIS 
149 
reduce them to flat platforms a little below the lowered sea level; and 
finally, when a rising temperature melts the continental ice-sheets and 
the sea surface is raised and warmed, and the corals are permitted to grow 
again, reefs are built up to the present sea level around the margin 
of the abraded platforms, producing barrier reefs or atolls as the case 
may be, around the lagoon that covers the abraded platform. The 
embayments of the central island within a barrier reef are explained 
as drowned valleys that were eroded while the sea level was lowered. 
After careful consideration I have had to discard this theory, except 
insofar as it may have produced small results that are altogether sub- 
ordinate to the larger effects of some more efficient cause. My reasons 
are in brief that, if the lagoons of large atolls have been abraded across 
their whole diameter of 20 or 30 miles, the central volcanic islands within 
narrow-lagoon barrier reefs should have been strongly cliffed by the 
lowered sea all around their shores, and their lagoon waters should 
now rise on the cHlBfed spur ends; but this is not the case; the spurs gen- 
erally dip gently into the lagoon with small cliffs or none. Further, 
if the embayments of the central islands within barrier reefs occupy 
new-cut valleys that were eroded during the lowered sea-stand of the 
glacial period, the up-stream parts of such new valleys should be visible 
beyond their embayed parts, and should there appear as incisions be- 
neath the floors of preglacial valleys, producing a valley-in-valley land- 
scape; but in the hundreds of embayments that I saw, no such composite 
valleys occurred. Finally, it is doubtful if the lowering of sea tem- 
perature generally sufficed to kill the corals and expose the reef flanks 
unprotected from sea attack; for on the atolls of the Paumotus Agassiz 
found many instances of slightly upHfted reef limestones, which he 
regarded as 'Tertiary,' and hence as preglacial, on the inner border of 
the present encircling reefs; and in such cases the lagoon floors could 
not be the result of marine abrasion in glacial time. 
Submergence and Subsidence. It might now appear as if no other 
cause than subsidence, as postulated in Darwin's theory and appar- 
ently confirmed by Dana's explanation of embayed shoreKnes, re- 
mained available; yet all the observable facts of the case may be fully 
as well explained by a rise of the sea surface, caused by an upheaval 
of the sea-bottom elsewhere, as by a sinking of the islands and of the 
sea-bottom on which they stand; but a deliberate discussion of this 
alternative shows it to be highly improbable because it demands extrav- 
agant crustal deformation and because it involves all the coasts of all 
the continents as well as the coral-reef islands. If it be set aside, only 
