44 



Ascension. 



PART I. 



the manner here supposed. Geologists have remarked, 

 that the external form of a bomb at once bespeaks the 

 history of its aerial course, and we now see that the 

 internal structure can speak, with almost equal plainness, 

 of its rotatory movement. 



M. Bory St. Vincent l has described some balls of 

 lava from the Isle of Bourbon, which have a closely 

 similar structure; his explanation, however (if I under- 

 stand it rightly), is very different from that which I 

 have given ; for he supposes that they have rolled, like 

 snow-balls, down the sides of the crater. M. Beudant, 2 

 also, has described some singular little balls of obsidian, 

 never more than six or eight inches in diameter, which 

 he found strewed on the surface of the ground : their 

 form is always oval ; sometimes they are much swpllen 

 in the middle, and even spindle-shaped : their surface 

 is regularly marked with concentric ridges and furrows, 

 all of which on the same ball are at right angles to one 

 axis : their interior is compact and glassy. M. Beudant 



supposes that masses of lava, 

 when soft, were shot into the air, 

 with a rotatory movement round 

 the same axis, and that the form 

 and superficial ridges of the 

 bombs were thus produced. Sir 

 Thomas Mitchell has given me 

 what at first appears to be the 

 half of a much flattened oval ball 

 of obsidian ; it has a singular 

 artificial-like appearance, which 

 volcanic bomb of obsidian from is well represented (of the natural 

 E size) in the accompanying wood- 

 cut. It was found in its pre- 



No. 4. 



a side view of the same ob- 

 ject. 



1 'Voyage aux Quatre Isles d'Afrique,' torn. i. p. 222. 



2 ' Voyage en Hongrie,' torn. ii. p. 214. 



