ORIGIN OF LITHOLOGICAL FEATURES. 377, 
none to form augite; and in other cases only part may be thus ab- 
sorbed (this probably depending on the proportional amount of silica 
present, and the temperature*), the rest, at a higher elevation in the 
lava conduit, going to the formation of augite or some allied compound. 
The various points in the problem appear to have been met and 
explained by reference to actual facts. ‘The basaltic mountain with a 
feldspathic centre, and the different constitution of fissure and crater 
eruptions, are necessary results of the general laws of physics; and 
we are not compelled to suppose the feldspar interior to have been 
formed first or last. ‘The whole is proved to be one single operation, 
governed by principles illustrated in the ordinary processes of the 
laboratory. 
The production of syenite is but a consequence of the same facts. 
The feldspathic centre is enclosed within a thick covering of rocks, 
and will therefore cool slowly; and though generally forming only dis- 
seminated crystals of feldspar in an earthy base, the cooling is some- 
times sufficiently gradual to allow of the whole crystallizing; and in 
this case, the texture throughout is crystalline, and the rock much re- 
sembles a granite. Under the same circumstances (or even a less 
gradual cooling) the elements of augite present will crystallize as 
hornblende: for these two minerals are identical except in crystalli- 
zation; and this difference depends on temperature and rate of cool- 
ing, hornblende requiring the slower rate. ‘Thus we have crystals of 
* Chrysolite contains usually about 41 per cent. of silica, with 59 of magnesia and prot- 
oxyd of iron. Augite consists of about 54 per cent. of silica with 46 of magnesia, lime, 
and protoxyd of iron, the iron often amounting to 6 per cent., and sometimes to 10 or 12, 
The following are analyses of each. 
CERYSOLITE. AUGITE. 
From Mount Somma. From Etna. From Vesuvius, 
Silica, - - - 40-08 50°55 50:90 
Lime, - - - cc 22°29 22°96 
Magnesia, - - 44°22 13°01 14°43 
Protoxyd of iron, - 15°26 7°96 6°25 
Protoxyd of manganese, 0:48 — 
Alumina, - - 0°18 4°85 5°37 
The analysis of chrysolite is by Walmstedt, A. Vet. Acad. Handl., 1824, ii. 359 ; the 
two of augite are by Kudernatsch, Poggend. Ann, xxxvil. 577. 
The formation of chrysolite at all in a volcanic focus must therefore be determined by 
the presence of a large proportion of magnesia compared with the amount of silica; and 
also by the temperature and pressure. ‘The conclusions in the text require to be farther 
tested by experiments on the crystallization of chrysolite. 
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