56 THE BUILDING OF AN ISLAND. 



upper beds. This formation of layers is especially conspicuo'us in the member 

 of the Trap group known as basalt, a hard, fine-grained black, sometimes 

 grey, rock, found in many parts of the world, as, for example, in the north- 

 east of Ireland and southwest of Scotland. It is over a layer of basalt that the 

 Zambesi in South Africa flows as it approaches the famous Victoria Falls, 

 about which it was formerly supposed that the wonderful cross-chasm into 

 which the river pours was produced by a great convulsion of Nature, but in 

 regard to which a recent observer has shown that both the chasm and the zig- 

 zag gorge, through which the river makes its way to the lower plain, have 

 been formed by the wearing action of the river itself, whose waters have cut 

 down through the basalt, the flow having followed great cracks which pass 

 across it. There do not appear to be any extensive overflows of basaltic or 

 other trap rocks in our islands, nor does basalt seem even to exist here, for 

 the hard black fragments which are sometimes found in the blue-beach con- 

 glomerates, especially in St. Thomas, are spoken of by Professor Cleve as " dark 

 porphyry or felsite." 



Questions as to the names and characters of the igneous rocks ai-e, how- 

 ever, for professional geologists rather than for amateurs, who. unless they 

 have special knowledge, must be satisfied with the more general results of their 

 observations. 



