88 ■ ERNEST H. L. SCHWARZ 



substance of the mud. In view of the presence of granite blocks 

 in the lavas, it is quite possible that some of the tuffs on these vol- 

 canic islands will turn out to be of the same nature as the granitic 

 tuffs such as are found in the Drakensberg and the Ries in Germany. 



In the Atlantic islands that do not stand on the central ridge 

 continental types of rocks are also found, as, for instance, in the 

 Canary Islands, where gneiss occurs; in the island of Mayo, -in the 

 Cape Verde group, Doelter found compact limestones and crys- 

 taUine schists — traces, he maintains, of an ancient continent.^ 



On the whole, therefore, the evidence of a land connection be- 

 tween Africa and South America, afforded by the islands in mid- 

 ocean, is suggestive, but far too httle investigated to be worth much 

 at the present time. The oceanic islands are not favorably situ- 

 ated for lengthened research; yet a very serious lacuna will exist 

 in our knowledge of the geology of the earth as a whole, if they 

 are not systematically studied. The surprising results of Wool- 

 nough in the Pacific makes one expect similar results from the Atlan- 

 tic islands; and then there are the islands in the Indian Ocean, 

 about which we know very little indeed, and the Aleutian Islands 

 in the north Pacific, which are similarly unexplored. I urge, there- 

 fore, that these oceanic islands can and do afford evidence of the 

 sub-oceanic crust, and if we are to understand the relation of our 

 present continents to the waters that encompass them, we must 

 not rely solely on the facts as presented to us in the most convenient 

 places, round our homes, but we must put off from our shores and 

 seek knowledge in isolated rock-masses of the ocean. 



In the second line of inquiry we can consider the evidence from 

 the American or the African side where the land bridge is sup- 

 posed to have emerged. 



In South Africa in Silurian times we have a remarkable band 

 of sandstone 5,000 feet thick, coarsely false-bedded throughout, 

 stretching from the west coast at a point about midway between 

 the mouth of the Orange River and Cape Town, to Port Ehza- 

 beth, and then coming in again at St. John's River, quite similar 

 in constitution, and passing eastward to Natal. This was suc- 

 ceeded by deeper water deposits containing characteristic Ameri- 



I Verhandlungen der K.-K. Geologischen Reichsanstalt (Wien, 1881), p. 16. 



