621 
in height more by the repeated accumulation of fresh voleanie 
products than they lose by the slow process of plastic subsidence. 
By way of summary, it seems to me that the following conclusions 
are justified : 
In order to explain the formation of barrier-reefs and atolls it is 
imperative to accept that a subsidence of the land (incl. islands) 
with respect to the sea-level took place in those regions where they 
occur, and to explain the formation of very many of these reef- 
structures in oceanic regions it is equally urgent to look upon this 
subsidence of the land with respect to the sea-level as having been 
considerable and to have extended over a long period. 
Three cases may be distinguished : 
1. The subsidence is the result of crustal movements. Such 
crustal movements will have been the ruling factors where barrier- 
reefs are found along the coasts of continents or where barrier-reefs 
and atolls are found encircling islands whiel on account of their 
structure and composition are closely related to neighbouring con- 
tinents, i.e. in the not truly oceanic regions, as e.g. the whole south 
western part of the Pacific. These crustal movements *) have very 
likely always had a compensatory character, subsidence of a certain 
amount in one region being compensated by upheaval of corre- 
sponding amount in neighbouring regions and vice versa; 
2. The subsidence of the land is only apparent and caused by 
positive movements of the sea-level, such as e.g. must have taken 
place in the late and _ post-pleistocene periods and probably still 
continue to some extent at the present day as the result of the 
melting of icecaps which in the pleistocene glacial period had 
accumulated on the continents, the importance of which movements 
for the origin of numerous barrier-reefs and atolls has been convin- 
cingly shown in Daty’s repeatedly mentioned theory ; 
3. The subsidence of the land is a real one, caused by the plastic 
yielding of isostatically non-compensated parts of the terrestrial crust 
in true oceanic regions under the influence of gravity, just as, accord- 
ing to the theory of isostasy, must be expected to occur in all true 
oceanic volcanic islands. This subsidence may be very considerable 
and will in fact only stop when such an island has entirely or 
nearly sunk away into the sima of the ocean-bed or rather will 
1) The’ author, besides the generally accepted orogenetic and epirogenetic move: 
irents, also admits the possibility of horizontal movements of continental blocks 
such as WEGENER assumes in his bold hypothesis about the origin of the con- 
tinents and oceans. 
A. WEGENER, Die Entstehung der Kontinente und Oceane. 1915. Kapitel 5 und 6, 
