

s- 



s 



^ 



volcanic bombs: ascension. 



155 



i w, 



M 



( 



so symmetrical round the entire circumference, that one is 

 forced to suppose that the bomb burst during its rotatory 

 course before becoming quite solidified, and that the lip and 

 edges were thus slightly modified and turned inwards " 



Darwin « On Volcanic Islands," pp. 38 and 39. 



86.— Fragment of a spherical volcanic bomb, with the 

 interior parts coarsely cellular, coated by a concentric layer 

 of compact lava, and this again by a crust of finely cellular 

 rock, forming the external surface. « This structure may 

 be explained, by supposing a mass of viscid scoriaceous 

 matter to be projected with a rapid rotatory motion through 



r^J ''/I Whilst * he exteraal <™t from cooling became 

 olidmed^ 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 

 di become, the nearer they were to this part, smaller and 

 smaller or less expanded until they became packed into a 

 hard sohd concentric shell. "-Darwin « On Volcanic Islands," 







v.;.: 



Pp. 36 and 37. 



87. 



t» ft Part ,° f A V0LCANIC B0MB of a similar description 

 o the preceding specimen, and showing the internal struc . 



ture. 



88. 





4, 



and fi !°? a V0LCANIC B0MB > composed of coarse 



and fine r cellular rock, of an irregularly scoriaceous struc- 

 ture: probably the central portion of the bomb. 



red t~ EjE f , E ? GRANITIC FRAGMENT > consisting of a brick- 



f«L , Par ' quartz ' and smaU dark Patches of a 



used mineral ascertained by its cleavage to be hornblende. 





V 



90. 



) 



{syenite) 



xZt Wi * red ' and """l**** "( white potash felspar, 

 IZT grain3 0f 1""^ and -»n crystals of horn. 





blende. 

 91. 



(wh 



felspa 



a dart ^i Y — " uuc ^ M /" (r . witn uttle nests of 



** wis zi r era i- often carious ' externa »y ™^, 



»»h „„ dlstlnct clea7age; probaMjr hornllen . 



Upper 

 Gallery, 



Wall-case 



1 



