404 W. M. DAVIS 
Large-scale ocean charts show that shallow platforms or banks do 
surround a number of extra-tropical volcanic islands, but the re- 
corded depths do not correspond to the depths that abraded plat- 
forms must have, if they exist, beneath the great submarine banks 
of the coral seas. Just as the central depths of coral-crowned 
banks are the most significant measures there offered, so the central 
depths of extra-tropical banks—or the depths close around their 
residual, cliff-rimmed islands—are also the most significant, because 
in both cases these central depths give the nearest indication of 
the attitude of sea-level when abrasion was taking place. It is 
true that the central surface of abrasion may have been worn 
somewhat below sea-level and that 1t may have been since then 
somewhat aggraded; but as both these variations may have similar 
values in all cases, it remains true that the central depths are the 
most significant for our purposes. The following examples may 
be cited. 
Two small volcanic islands, Bird and Necker, in the north- 
western extension of the Hawaiian group, have been described by 
Elschner’ as rising with strongly clift borders of resistant lavas 
from extensive banks, one or two score miles across. Bird Island, 
latitude 23° N., is goo feet high; its bank has depths of from 12 
to 20 fathoms near the island; Necker Island, latitude 235° N., 
has a height of 300 feet; its bank has depths of less than 10 fathoms 
for a quarter-mile from its cliffs. Both banks deepen to 40 or 50 
fathoms at their borders. It is noteworthy that coral reefs occur 
not far away, and it is therefore eminently possible that reefs may 
have been formed on these island banks also, either in preglacial 
time or in an interglacial epoch, the latest of which was, according 
to glacialists, longer and warmer than the present postglacial 
epoch. But if this be the case, the presence of strong cliffs on the 
residual volcanic islands here makes the absence of spur-end cliffs 
on the reef-encircled islands of the torrid seas all the more 
significant. 
In the South Pacific, Norfolk Island, a volcanic mass in latitude 
29. S., between New Zealand and Australia, is strongly clift, 
1 C. Elschner, “The Leeward Islands of the Hawaiian Group.’ Reprinted from 
the Sunday Advertiser, Honolulu, 1915. 
