625 
corals, namely Bermuda’), it has recently been proved (see above) 
that the reef limestones which nowadays project there above the 
sea are the upper portion of a cap or crown of reef-limestone, which 
is at least 110 m. thick and rests on a sunken basaltic mountain. 
All other true oceanic voleanie islands in the Atlantic lie outside 
the area of the reef-building corals. They are all voleanic 2) 
and are situated on the so-called mid-Atlantic ridge (“Mittelatlan- 
tische Bodenschwelle’”’). The Canary and Cape Verd Islands certainly 
and the Selvagen Islands and the Madeira Group probably are not 
true oceanic islands, but have once formed part of the European- 
African continent *). 
Perhaps we may see in this remarkable mid-Atlantic ridge the 
final result of volcanic activity along an enormous fracture of 
the same extent, where from numerous fissures and vents volcanic 
material was discharged, thus a voleanie mountain chain and cones 
being formed, which nowadays subside through yielding under the 
influence of gravity and nearly all have sunk back to a level 
approaching the average level of the deep submarine ridge. Here 
and there a few islands, where volcanic activity lasted longer or has 
existed to this dav, still rise above the sea‘) and others (of which 
1) The West Indian archipelago proper with its numerous coral islands and 
reef-formations does not belong to the group of true oceanic islands. West India 
is a region of strong and recent diastrophism, intimately related to the American 
continent. 
*) GAGEL mentions the occurrence of numerous loose boulders of gneiss and 
granite on Santa Maria, one of the Azores, and adds that these rocks are not 
indigenous there, but problably have been carried thither during the pleistocene 
glacial epoch by icebergs. CG. GAGEL, Die mittelatlantischen Vulkaninseln. Handb. 
der region. Geologie VIl. H. 10, p. 12, 1910. 
The evidence considered as acceptable by Scuwarz for the occurrence of non- 
voleanic (continental) rocks in some of these islands, is, to quote his own words, 
“not so good as one could wish for, and could not be admissible were the islands 
more easy of access, or had a geologist been to the place himself”. 
E. H. C. Senwarz. The rocks of Tristan d’Acunha, brought back by H. M. S. 
Odin, 1904 with their bearing on the question of the permanence of ocean basins. 
Trans. of the S. A.’Phil. Soc. Vol. XVI, p. 9, 1905. 
*)' CG. Gace. |, €. pp. 31. 
4) The fact, that several of the volcanic islands in the Atlantic, as e.g. Nightingale 
island and Tristan d’Acunha, are very well cliffed, appears to afford a strong 
argument against my hypothesis which requires slow but continuous subsidence 
for these islands also. This argument, however, loses its strength, if it is borne 
in mind, that the process of cliffing by wave-action is a rapid one, especially on 
these volcanic islands, which are partly composed of incoherent or little coherent 
ejacamenta (or efflata), and, their coasts not being protected by fringing reefs, are 
exposed on all sides to the fuil fury of the mid-ocean waves. The process of sub- 
40) 
Proceedings Royal Acad. Amsterdam. Vol. XIX. 
