1883.] 477 [Crosby. 



certainly unreasonable to say that all oceanic islands must be 

 remote from the continents. As well might it be claimed that all 

 the higher parts of the continents, or mountains, must be remote 

 from the sea. I have been informed by Prof. Jules Marcou that 

 the Marquesas Islands, lying on the eastern border of Polynesia 

 and near the centre of the Pacific, contain representatives of the 

 older stratified formations. And now we learn through Dr. C. 

 Doetter of Graz that the Cape Yerde Islands do not consist ex- 

 clusively of volcanic rocks, but contain also gneiss, mica and clay 

 slates, and limestones. 1 Paleozoic and Mesozoic rocks are well 

 developed in Spitzbergen, which, it would seem, may be fairly 

 classed as an oceanic island. Again, many oceanic islands have 

 not been examined geologically with sufficient care to justify 

 Mr. Wallace's sweeping and positive statement that, with two 

 exceptions, none of them contain any traces of the older stratified 

 formations. 2 



With the exceptions noted, the oceanic islands are nearly all 

 small, and are composed of eruptive rocks or of coral reefs resting, 

 presumably, upon a volcanic foundation. The oceanic islands are, 

 of course, merely the tops of submerged mountains ; and it is only 

 with the highest points of the continents that they can be prop- 

 erly compared. Now, supposing the existing continents were 

 submerged to an average depth of 15,000 feet, what would be the 

 geological character of the land remaining above the sea? Pal- 

 eozoic and Mesozoic rocks would probably be about as scarce in 

 it as in modern oceanic islands. As a rule the loftiest mountains 

 of the globe are composed of eruptive rocks, and in many cases 

 they are distinct, or even active, volcanoes ; although the main 

 mass of every mountain system is formed of stratified, and often 

 of fossiliferous, formations. The volcanic materials usually form 

 but a small part of the whole ; but they are the cap-sheaf. 



i Petermann's Mittheilungen, 1883, 72. 



2 The report of the German Meteorological Expedition to South Georgia Islands 

 contains the interesting information that the only rock observed on this ice-capped 

 Antarctic land is clay -slate. Even the blocks brought down by the glaciers from 

 the lofty mountains of the interior were all slate. (Nature, 29, 509.) And Mr. T. 

 Mellard Eeade (Geol. Mag. I, 225), in commenting on this report, directs attention to 

 the fact that this large remnant of an undoubtedly ancient and extensive sedimentary 

 formation is now a true oceanic island, standing in deep water remote from the conti- 

 nents. 



