619 
These islands as far as they are not composed of reef-limestone, 
are found to consist, exclusively of voleanic material, as a rule of 
basaltic or allied rocks’). The non-volcanie or not entirely voleanic 
islands in the Pacific are restricted to the south-western part which 
stands in close relation to the Asiatic and Australian continents, 
while on the other hand not only in the Pacific, but also in the 
Atlantic all true oceanic islands are composed of volcanic rocks. 
Such true oceanic islands, as far as they have been studied, are 
not isostatically compensated and, without exception, show a larger 
or smaller positive anomaly of gravity *). 
The position of these islands is exceptional, resting immediately 
on the sima and being rooted into it, and not being supported by 
much larger masses of salic material, by continental blocks, as is 
the case with the incompletely-compensated or non-compensated 
nuclei in and on the continents. 
It appears to me that, on account of the isostasy itself, these 
voleamie islands, rising directly from the plastic sima as cones or 
groups of cones of considerable bulk, cannot always remain in 
existence*®); under the influence of gravity they will without exception 
yield and sink down slowly but gradually and if this movement is 
not counteracted by other forces they must disappear below the sea 
and finally approach more and more the form of the ocean-bed, 
being welded again with and incorporated in the sima below the 
ocean bottom. 
In such islands conditions must be exceptionally favourable for 
the formation of barrier reefs and in case of total submergence, of 
tj The reader is referred to: G. GERLAND. Vulkanistische Studien. |. Die Korallen- 
inseln der Südsee. Beiträge zur Geophysik. IL. 1895, p 29—34, where in a 
convincing discussion it is rendered all but certain that all corai islands of the 
central Pacific rest on a volcanic base; and A. A. Dany. Problems of the Pacific 
Islands. Amer. Journal of Science XLI, 1916, where on p. 153 it is pointed out 
that the statements about non-voleanie continental rocks occurring in some of these 
islands are not certain and require revision. 
2) E. Borrass. Bericht über die relativen Messungen der Schwerkralt mit Pen- 
delapparaten in der Zeit von 1808 bis 1909. Verh. der 16ten Allg. Conferenz der 
internat. Erdmessung I]. Berlin 1911. 
8) WEGENER (I. c. p. 13) probably had this idea in his mind and it evidently 
led him to suppose that these volcanic oceanic islands should in reality be isosta- 
tically compensated, although the contrary has been proved, and that they could 
not wel! be entirely composed of volcanic material but ought to contain a nucleus 
of salie (continental) material, which should be relatively very large, since about 
950/, of it must be hidden under the sea-floor immerged in the sima. In my 
opinion these suppositions being, contrary to all observed facts, must be regarded 
as exceedingly improbable. 
