18 Transactions of the South African Philosophical Society. 



Michael's in the Azores ; and the Falcon Island in the Tonga group ; 

 as well as the sand island in Walfisch Bay, so ably described by 

 Mr. Waldron in the Transactions of this Society, is clear enough 

 evidence. The volume of the ocean is so immense, however, that 

 the amount of contraction necessary to raise the surface level 100 

 fathoms, is beyond what we have observeeMio be the possibilities of 

 earth-movements. 



There seems to me to be a very simple explanation of the con- 

 tinental shelf, if we assume the correctness of the principle of 

 isostatic equilibrium, that is, that if the crust of the earth be 

 weighted with sediment at one place and relieved of a load at 

 another by denudation, there will be a sinking at the former and a 

 rising of the latter. Applying this to the case in question, the 

 interior of the continent being rapidly eroded and washed away, and 

 the materials that once went to form the mountains are being 

 deposited off the shore as sediment, there will, therefore, be a tilting 

 seawards, the coastal regions will be submerged while the interior 

 will rise. This is no new idea that is unsupported by any clearer 

 evidence than what I have just given ; it can be actually seen in 

 progress in South America where the tilting of the continent towards 

 the west was recognised when the first account of its geology was 

 brought home by Charles Darwin. 



Other examples of oceanic islands containing continental types of 

 rock are those of the Falkland Islands, South Georgia, and Trinidad, 

 but these, as clearly as the Porcupine Bank and the Bockall 

 Island, are situated on the continental plateau, and their geology, 

 therefore, excited no great attention. 



The Falkland Islands are especially interesting. They contain 

 volcanic rocks surprisingly like those of Tristan d'Acunha and our 

 Cave Sandstone ; primarily, however, they are made of rocks which 

 appear to belong to the same series as our South African Witteberg 

 and Bokkweld beds ; at any rate, the fossils that have been brought 

 from there are such as might have been collected in the Colony — at 

 Ceres, for instance. The type to which this fauna conforms to is 

 decidedly unlike the European Devonian fauna ; it is characterised 

 by the Brachiopod, Leptocoelia flabellites, Conrad, the Orthis 

 palmata of Morris and Sharpe, a form which, associated with Vitu- 

 lina j)iistulosa, Hall, a number of Spirifers, Conularias and Trilobites 

 of peculiar species, is found throughout North and South America in 

 certain zones of the Devonian." We thus get an assemblage of 



* Conrad, 5th Ann. Kep. N.Y. Geol. Survey, 1841, p. 55 ; Morris and Sharpe, 

 Q.J.G.S., II., 1846, p. 276; Hall, Palseont. N.Y., 1859, vol. iii. p. 449 

 Billings, Geol. Canada, 1863, p. 369 ; Meek and Worthen, Geol. Snrv. Illinois, III., 



