SiS Notices respecting New Booh : — Darwin on the 



which presented a symmetrical and very curious appearance, of which 

 he gives a figure and the subjoined description : — 



" The whole interior is coarsely cellular ; the cells averaging in diameter 

 about the tenth of an inch ; but nearer the outside they gradually decrease 

 in size. This part is succeeded by a well-defined shell of compact lava, 

 having a nearly uniform thickness of about the third of an inch ; and the 

 shell is overlaid by a somewhat thicker coating of finely cellular lava (the 

 cells varying from the fiftieth to the hundredth of an inch in diameter), 

 which fornis the external surface : the line separating the shell of compact 

 lava from the outer scoriaceous crust is distinctly defined. This structure 

 is very simply explained, if we suppose a mass of viscid scoriaceous matter 

 to be projected with a rapid, rotatory motion through the air ; for whilst the 

 external crust, from cooling, became solidified (in the state we now see it), 

 the centrifugal force, by relieving the pressure in the interior parts of the 

 bomb, would allow the heated vapours to expand their cells ; but these 

 being driven by the same force against the already-hardened crust, would 

 become, the nearer they were to this part, smaller and smaller or less ex- 

 panded, until they became packed into a solid, concentric shell. As we 

 know that chips from a grindstone * can be flirted off, when made to revolve 

 with sufficient velocity, we need not doubt that the centrifugal force would 

 have power to modify the structure of a softened bomb, in 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 in- 

 ternal structure can speak, with almost equal plainness, of its rotatory move- 

 ment." 



M. Beudant has described in his Travels in Hungary, some sin- 

 gular little oval balls of obsidian, found strewed on the surface of the 

 ground, their surface regularly marked with concentric ridges and 

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

 axis ; and he supposes that these were produced by masses of lava, 

 which when soft were shot into the air with a rotatory movement 

 round the same axis. This leads Mr. Darv^^in to describe and figure 

 a volcanic bomb of obsidian from Australia, which exhibits the ex- 

 ternal structure described by M. Beudant, and the internal cellular 

 condition of the bombs from Ascension, described in the preceding 

 extract. In this case, from certain parts of the existing structure, 

 one is forced to suppose, as the author remarks, that the bomb burst 

 during its rotatory course, before being quite solidified, and that the 

 axis of rotation then changed. 



The following observations relating also to some of the rocks of 

 Ascension are important, as involving the history of a process partly 

 related to that by which siliceous petrifactions have been produced, 

 a subject as yet enveloped in total obscurity : — 



"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, 



* '• Nichol's Architecture of the Heavens." 



