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206 DYNAMICAL GEOLOGY. © [Boox III. 
find among them pieces of non-volcanic rocks as well as of older tuffs 
and lavas recognisably belonging to early eruptions. In many cases | 
they are ejected in enormous quantities during the earlier phases of 
violent eruption. The great explosion from the side of Ararat in 
1840 was accompanied by the discharge of a vast quantity of 
fragments over a space of many square miles around the mountain. 
Whitney has described the occurrence in California of beds of such 
fragmentary volcanic breccia hundreds of feet thick and covering 
many square miles of surface. Junghuhn in his account of the 
eruption in Java in 1772, mentions that a valley ten miles long 
was filled to an average depth of fifty feet with angular volcanic 
debris.’ 
Among the earlier eruptions of a volcano fragments of the rocks 
through which the vent has been drilled may frequently be observed. 
These are in many cases not volcanic. Blocks of schist and 
eranitoid rocks occur in the cinder-beds at the base of the 
voleanic series of Santorin. In the older tuffs of Somma pieces 
of altered limestone are abundant and often contain cavities 
lined with the characteristic “ Vesuvian minerals.” Blocks of a 
coarsely crystalline granitoid lava have been particularly observed 
both on Etna and Vesuvius. In the year 1870 a mass of that kind, 
weighing several tons, was to be seen lying at the foot of Vesuvius, 
within the entrance to the Atrio del Cavallo. Similar blocks oecur 
among the Carboniferous volcanic pipes of central Scotland, together 
sometimes with fragments of sandstone, shale, or limestone, not 
infrequently full of Carboniferous fossils.? 
(4) Volcanic Bombs and Slags.—These have originally 
formed portions of the column of lava ascending the pipe of the 
volcano, and have been detached and hurled into the air by the 
successive explosions of steam. A bomb (Fig. 34) is a round, 
elliptical, or pear-shaped, often discoidal mass of lava, from a few 
inches to several feet in diameter ; sometimes tolerably solid through- 
out, more usually coarsely cellular inside. Not infrequently its 
interior is hollow, and the bomb then consists of a shell which 
is most close-grained towards the outside. There can be no 
doubt that, when torn by eructations of steam from the surface 
of the boiling lava, the material of these bombs is in as thoroughly 
molten a condition as the rest of the mass. From the rotatory 
motion imparted by its ejection it takes a circular form, and 
in proportion to its rapidity of rotation and fluidity is the amount 
of its “flattening at the poles.” The centrifugal force within 
allows the expansion of the interstitial vapour, while the outer 
surface rapidly cools and solidifies; hence the solid crust, and 
the porous or cavernous interior. Such bombs, varying from the 
size of an apple to that of a man’s body, were found by Darwin 
abundantly strewn over the ground in the Island of Ascension ; 
' But see the remarks already made on voleanic conglomerates, ante, p. 163. 
* Trans. Ioy. Soc, Edin, xxxix, p. 459. Sve postea, Book IY. section vii. § 1, 4. 
