20 EXCURSIONS IN MADEIRA 



toward the sea between this point of the diff, and that close to 

 the Pontinha, on the east. Of this we may easily satisfy ourselves, 

 by examining the direction of the basalt ridges which mark the 

 course of the streams, and the surface of the country behind, and 

 level with the cliff inland. For some distance, the cliff presents 

 nothing but the beds of tufa, scoriae, and pumice, in short, precisely 

 the same as the section drawn, taking away the beds of columnar 

 basalt; but a slip on the eastward terminates in the appearance of 

 scoriaceous basalt, beneath the yellow tufa, which has hitherto 

 formed the lowest bed of our section, and that, as if it were a 

 prolongation of the band of scoriae, and had forced its way through 

 the yellow tufa, thus. 



The breadth of this stream of scoriaceous basalt, is about seventy 

 yards ; the depth of the upper, or vaulted part, as seen in the 

 drawing, Plate 3, B, is about twenty feet, that of the lower part, 

 (which is composed of sheets and ridges running into the sea, and 

 dipping to the south, in an angle of 20°) is seldom more than four 

 feet. Beneath this scoriaceous basalt is red, passing into yellow 

 tufa, with a band of pumice, at the lowest visible part of the bed, 

 which is about ten feet deep when it is lost sight of: this lowest 

 tufa, scarcely discoverable in the cavernous part of the scoriaceous 

 basalt, is best seen in the break close to the left of the hut, in the 

 drawing. 



The cells, of which this lower, or scoriaceous basalt is full, are 

 generally long and narrow ; in a perpendicular section they appear 

 confused, but when the basalt has been cleft in an inclined plane, 

 parallel with its dip, it becomes evident, that these cells, (always 



