

GEOLOGY OP FERNANDO ffOBONHA 



89 



sometimes, in fact, the whole height of the cliff, but are not very 

 broad. 



Scorias, Pumices, and Tuffs. 



At the eastern end of the island is a large quantity of a red 

 clayey soil, covered in part by some sand-dunes, and apparently 

 overlying the ordinary basalt. This appears to be a scoriaceous 

 basalt which has been much altered by the action of acid vapours. 

 Another large band, harder in texture, runs from the low hills 

 on the south of the road from San Antonio to Chaloupe Bay, 

 where it crops out between two masses of basaltic rocks ; it is 

 of considerable depth and 100 yards in width. The central 

 plateau of the island is covered also with a somewhat similar red 

 clay-like deposit, which is very fertile and thickly covered with 

 fodder-plants &c. This appears to be a ferruginous scoriaceous 

 rock, probably poured out from a volcano in the form it is at 

 present, and not the product of decomposition of basalt, as it at 



first 



appears. 



At Look-out Hill, Boldro, and other places where the basalt 

 comes in contact with the phonolite and has not been much com- 

 pressed, it was loose in texture and frequently amygdaloidal, with 

 zeolites. 



Between Morro branco and Point Noir was a very interesting 

 series of volcanic rocks. After passing the lake, which is sur- 

 rounded by a high semicircle of cliffs of basalt arranged in layers, 

 we come to a promontory consisting of an immense barren mass 

 of large fragments of basalt, irregular in size and shape and piled 

 on the top of each other to a great height above the sea. They 

 appear to be broken columns of fine-grained basalt, and are known 

 as Pedras Pretas (the Black Eocks). Beyond these is an indenta- 

 tion terminated at the west by Cape Noir, a black cape of basalt. 

 Between the Cape and Pedras Pretas is a steep slope, about 700 

 feet high, consisting of a thick bed of scoria, in the shape of rolled 

 balls overlain by a thick bed of basalt in layers. The scoria-bed 

 was not flat, but was steeply sloped from the top of the hill to 

 the level of the sea, the basalt following the curve. Here, we had 















thrown 



K' 



nipt 









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which still retained the slope at which they eventually settled 

 when falling from the crater and were then covered by the flows of 

 lava which followed this first eruption, and which has preserved 





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