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204 DYNAMICAL GEOLOGY. —— [Boox III. 
vapours. Some appear to be saturated, others contain a much 
smaller gaseous impregnation; and hence arise important dis- 
tinctions in their behaviour (pp. 218, 224). After solidification lavas 
present some noticeable characters then easily ascertainable. (1) 
Their average specific gravity may be taken as ranging between 
2:37 and 3°22. (2) The heavier varieties contain much magnetic 
or titaniferous iron, with augite and olivine, their composition being 
basic, and their proportion of silica averaging about 45 or 50 per 
cent. In this group come the basalts, dolerites, nepheline-lavas, 
and leucite-lavas. The lighter varieties contain commonly a minor 
proportion of metallic bases, but are rich in silica, their percentage 
of that acid ranging between 60 and 80. They are thus not 
basic but acid rocks. Among their more important species trachyte, 
rhyolite, obsidian, pitchstone, and pumice may be enumerated. Some — 
intermediate varieties (augite-andesite, hornblende-andesite) connect 
the acid and basic series. (3) They differ much in structure and 
texture. (a) Some are entirely crystalline, consisting of nothing 
but an interlaced mass of crystals and crystalline particles, as in some 
dolerites, and granitoid liparites. Even quartz, which used to be 
considered a non-voleanic mineral characteristic of the older and 
chiefly of the plutonic eruptive rocks, has been observed in large 
crystals in modern lava as in liparite.and quartz-andesite.’ (b) Some 
show more or less of a half-glassy or stony matrix, in which the 
constituent crystals are imbedded ; this is the most common arrange- 
ment. (c) Others are entirely vitreous, such crystals or crystalline 
particles as occur in them being quite subordinate, and, so to speak, 
accidental enclosures in the main glassy mass. Obsidian or volcanic 
elass is the type of this group. (d) They further differ in the 
extent to which minute pores or larger cellular spaces have been 
developed in them. According to Bischof the porosity of lavas 
depends on their degree of liquidity, a porous lava or slag, when reduced 
in his experiments to a thin-flowing consistency, hardening into a 
mass as compact as the densest lava or basalt.? But the presence of 
interstitial steam in lavas, by expanding the still molten stone, 
erin an open cellular texture, somewhat like that of ill- 
aked bread. Such a vesicular arrangement very commonly appears 
on the upper surface of a lava current, which assumes a slagey 
or cindery aspect. (4) They vary greatly in colour and general 
external aspect. The heavy basic lavas are usually dark grey, or 
almost black, though, on exposure to the weather, they acquire a 
brown tint from the oxidation and hydration of their iron. Their 
surface is commonly rough and ragged, until it has been sufficiently 
decomposed by the atmosphere to crumble into excellent soil which, 
under favourable circumstances, supports a luxuriant vegetation. 
The less dense lavas, such as phonolites and trachytes, are frequently 
paler in colour, sometimes pale yellow or buff, and decompose into 
' Wolf, Neues Jahrb. 1874, p, 377. 
* Chem, und Phys. Geol, Supp. (1871), p. 144. 

