. AND PORTO SANTO. 21 



empty) are in the same direction as the stream. So abundantly 

 have the gases escaped in the parts near the surface, that the cavities 

 are considerable, probably from the cells left by the gases having 

 been so close and frequent as to communicate with each other, and 

 from their slight and imperfect partitions being subsequently 

 obliterated by decomposition. The sides of these ridges present 

 larger cells ; continued projecting ledges, twists, and folds ; and 

 look in every respect like igneous streams, suddenly petrified. 

 In the cavernous part (represented in the drawing) this scoriaceous 

 basalt, particularly open to the attack of destructive elements, from 

 its numerous cells, is in such an advanced state of decomposition, 

 that it looks like a dark brown indurated mud, in which more 

 obdurate masses are here and there imbedded. But the shallower 

 vaulted cavities, which we remark in the lower part of these ridges, 

 are probably owing to the heated stream of basalt having passed 

 over some patches of water left by the retiring tide, just before it 

 reached the sea, which, being immediately converted into vapour, 

 raised these vaults above the parts where they were developed and 

 liberated. This scoriaceous basalt, where it is not in an advanced 

 state of decomposition, is full as hard as the compact, but of a less 

 specific gravity''. 



Beai'ing in mind, that the streams of scoriaceous basalt presenting 

 these characters, have flowed directly into the sea, which still 

 washes over them, and that the columnar basalt crowns the tops 

 of the cliffs only, (which are from fifty to one hundred feet above 

 the level of the sea) it is evident, that sudden congelation, instead 

 of producing such an arrangement of particles, as M. Faujas sup- 

 posed, entirely prevents it'. We shall presently find too, that the 



T Compact basalt, 2.9. Scoriaceous basalt, 2.6 : by Guyton's glass Areometer, 

 and in distilled water. Red tufa, 1.75. Yellow tufa, 1.94. 



' In the natural section close to the Pontinha on the east, the columns are more 

 symmetrical than to the west; and the basalt, where it has been broken away by the 

 workmen, presenting transverse cracks, and a hackly appearance. Plate 3, C. 



