Observations on Volcanoes, 



Such are the external accidents of a simple volcanic cone. The 

 internal structure is stated as being conglomerated, and the 

 substances described are scoria and fragments. The scoria are 

 portions of the liquid lava projected into the air, and which being 

 consolidated in their transmission, fall down in various fantastical 

 shapes, resembling in their general aspect the scoria of furnaces, 

 and often called volcanic cinders. The rounded masses, called 

 volcanic bombs, often contain in the interior a fragment of some 

 rock, as a sort of nucleus, and they are generally more solid than 

 the others. Very liquid lava, of a low specific gravity, is supposed 

 to form pumice ; and if the specific gravity is higher, some other 

 species of cavernous structure ; while an inferior liquidity pro- 

 duces scoria, with few, or with rough and angular cells ; and it is 

 considered that the size and regularity of the air-bubbles vary 

 directly with the fineness of the grain. 



Scoria of all kinds are sometimes broken, or even ground into 

 dust, by the collisions, it is here supposed, which they undergo 

 in the air ; and different degrees of fineness give lapelli, pozzo- 

 lana, and volcanic sand and ashes. 



The other class of fragments consists of pieces of the hard rock 

 broken from the sides of the fissure by the force of the explosion ; 

 and these, of course, present great diversity of character, because 

 they may belong to many different strata. These fragments have 

 generally undergone considerable changes during their passage 

 through the fire, or their contact with it. Thus, some are in- 

 crusted with lava ; others are partially fused or burnt : while, in 

 some cases, they are supposed to have been more materially al- 

 tered in character by a re- combination of their elements. Such 

 fragments are rare, from obvious causes, in ancient volcanoes 

 erupting from old vents. The explosive force i3 so powerful as 

 to throw such fragments sometimes to the height of 4000, and 

 even of 6000 feet. 



The conglomerate structure of the volcanic cone is sometimes 

 irregular; while at others, the different substances are stratified, 

 and in modes successively parallel to the surface of the hill. But 

 it also necessarily happens that this stratification must be often 

 disturbed by commotions within the mountain, resulting from 

 other eruptions, with their consequent fissures and other causes ; 

 while further, the fragments are sometimes consolidated into a 

 rock by currents of lava. 



The fourth chapter examines the disposition of lava when 

 ejected. Felspar, titaniferous iron, and augite, are considered as 

 the chief constituents of these rocks. Their names are, according* 

 to the author, trachyte, phonolite, trachytic porphyry, compact 

 felspar, pitchstone, obsidian, basalt, dolerite, greenstone, basaltic, 

 clinkstone, and gallinace, to which he adds another, called gray- 

 Stone. The specific gravity of felspar to the other substances, is 



