a 
oc 

ra 
Parr I. Sect. i. § 1.] VOLCANIC PRODUCTS. 207 
they were also ejected in vast quantities during the eruption of 
Santorin in 1866.1 Among the tuffs of the Eifel region small 
bombs, consisting mostly of granular olivine, are of common 
occurrence, as also pieces of sanidine or other less fusible minerals 













































































































Fic. 34.—SEctTion oF VoLcaANnic Bomb, ONE-THIRD NATURAL SIZE. 
which have segregated out of the magma before ejection. When 
the ejected fragment of lava has a rough irregular form, and a 
porous structure like the clinker of an iron-furnace, it is known as 
a slag.? 
The fragmentary materials erupted by a volcano and deposited 
around it acquire by degrees more or less consolidation, partly from 
the mere pressure of the higher upon the lower strata, partly from the 
influence of infiltrating water. It has been already stated (p. 161) 
that different names are applied to the rocks thus formed. The 
coarse, tumultuous, unstratified accumulation of volcanic debris within 
a crater or funnel is called Agglomerate. When the debris, 
though still coarse is more rounded, and is arranged in a stratified 
form on the slopes of the cone or on the plain beyond, it becomes a 
Volcanic Conglomerate. The finer-grained varieties, formed 
of dust and lapilli, are included in the general designation of Tuffs. 
These are usually pale-yellowish, greyish, or brownish, sometimes 
black rocks, granular, porous, and often incoherent in texture. 
Organic remains sometimes occur in tuff. Where volcanic debris 
has accumulated over the floor of a lake, or of the sea, the entombing 
and preserving of shells and other organic objects must continually 
take place. Examples of this kind are cited in later pages of this 
volume from older geological formations. Professor Guiscardi of 
1 Geological Observations on Volcanic Islands, 2nd edit. p. 42. Fouqué, op. cit. 
p19. 
2 On the ratio between the pores and volume of the rock in slags and lavas, see 
determinations by Bischof, Chem. und Phys. Geol. Supp. (1871), p. 158. 
