The Bocks of Tristan d'Acunha. 17 



that are the same as those found on the mainland of Africa/ 1 ' The 

 explanation of this is fairly satisfactory, for it is very nearly certain 

 that in former ages there was a great extension of the Madagascar 

 ridge, probably assuming continental proportions, and this has now 

 by earth movements been dropped, and only the highest portions 

 left above sea-level, f 



It is quite legitimate to question whether the sea has not risen 

 instead of the land sinking, and as this is an important factor in the 

 following discussion it is worth while stopping to examine the argu- 

 ments for and against at this place. 



The latest upholder of the rising of the sea in some instances, as 

 against the sinking of the land, is Mr. Eeade, who adduces in 

 evidence the great extension of submarine plateaus that extend out 

 from the continents. The most notable examples are those on 

 either side of the North Atlantic. For some 150 miles off what is 

 now land in Europe, the plateau extends, having an average depth 

 of 100 fathoms ; on it are the elevations called the Eockall Island 

 and Bank, | and the Porcupine Bank, the latter only rising to 85 

 fathoms ; the one is 240 miles from the Irish mainland, the other 

 130 miles. Similarly off the American coast the shelf extends under 

 the sea for a great number of miles, and includes the Newfoundland 

 Bank. Professor Cole also seems to add confirmatory evidence from 

 his quoting the evidence of Nansen, that between Iceland and Jan 

 Magen Island there are sunken shell banks. § Mr. Eeade would 

 argue that it is improbable that two great land masses on either 

 side of the globe would simultaneously sink to the same extent, and 

 leave corresponding shelfs containing the land surface at approxi- 

 mately the same depth beneath the sea ; more probably, he says, 

 the sea- bottom was elevated, and the water overflowed the margin 

 of its former basin. 



In South Africa we have a similar shelf projecting into the sea, 

 whose edge is known as the Agulhas Bank ; and the question has, 

 therefore, interested me for a long tims. It is certain that the 

 volume of the ocean-basin can, and is, repeatedly altered by earth 

 movements going on on the sea-floor, the very fact of islands 

 suddenly appearing, such as those of Graham's Island, or He Julia, 

 between Sicily and the coast of Africa; Sabrina Island off St. 



* E. P. Wright on the Seychelles Islands, Eep. Brit. Ass. 1868, p. 143. 



f See " Volcanoes of GriqualandEast," Trans. Phil. Soc, vol. xiv. , Cape Town, 

 1903, p. 98. 



I J. W. Judd and T. Buperfc Jones, Trans. Boy. Irish Acad., vol. xxxi., 1897, 

 pp. 59 and 97. 



§ S. A. Cole and T. Crooke, Appendix No. IX. to Part II. of " Beport on the Sea 

 and Inland Fisheries of Ireland for the year 1901," p. 9 of author's reprint. 



2 



