52 Ascension. part i. 



ground, they occasionally extend some yards in length ; 

 these plate-like veins, when struck, emit a sound, almost 

 like that of a drum, and they may be distinctly seen to 

 vibrate ; their fragments, which are strewed on the 

 ground, clatter like pieces of iron when knocked against 

 each other. They often assume the most singular 

 forms: I saw a pedestal of the earthy trachyte, covered 

 by a hemispherical portion of a vein, like a great 

 umbrella, sufficiently large to shelter two persons. I 

 have never met with, or seen described, any veins like 

 these : but in form thev resemble the ferruQ-fnous seams, 

 due to some process of segregation, occurring not un- 

 commonly in sandstones, — for instance, in the New Red 

 sandstone of England. Xumerous veins of jasper and 

 of siliceous sinter, occurring on the summit of this 

 same hill, show that there has been some abundant 

 source of silica, and as these plate-like veins differ from 

 the trachyte only in their greater hardness, brittleness, 

 and less easy fusibility, it appears probable that their 

 origin is due to the segregation or infiltration of silice- 

 ous matter, in the same manner as happens with the 

 oxides of iron in manv sedhnentarv rocks. 



Siliceous sinter and jasper. — The siliceous sinter 

 is either quite white, of little specific gravity, and with 

 a somewhat pearly fracture, passing into pinkish pearly 

 quartz ; or it is yellowish white, with a harsh fracture, 

 and it then contains an earthy powder in small cavities. 

 Both varieties occur, either in large irregular masses 

 in the altered trachyte, or in seams included in broad, 

 vertical, tortuous, irregular veins of a compact, harsh, 

 stone of a dull red colour, appearing like a sandstone. 

 This stone, however, is only altered trachyte ; and a 

 nearly similar variety, but often honeycombed, some- 

 times adheres to the projecting plate-like veins, de- 

 scribed in the last paragraph. The jasper is of an ochre 



