266 - DYNAMICAL GEOLOGY. — 

Reference has already (p. 58) been made to the speculation of 
Durocher as to the existence within the crust of an upper siliceous _ 
Jayer with a mean of 71 per cent. of silica and a lower basic layer 
with about 51 per cent. of silica. Bunsen also came to the conelu- 
sion that volcanic rocks are mixtures of two original normal magmas | 
—the normal trachytic (with 67—76 silica, and a ratio of acid to 
base of 5 to 1), and the normal pyroxenic (with 47—48 silica and a 
ratio of 3 to 2 between acid and base). The varying proportions in 
which these two original extreme magmas have been combined are, 
in Bunsen’s view, the cause of the differences of voleanic rocks. We 
may conceive these two layers to be superposed upon each other, 
according to relative densities, and the composition of the last erupted 
at the surface to depend upon the depth from which it has been 
derived.1_ The earlier explosions of a volcano may be supposed to 
take place usually from the upper lighter and more siliceous layer, 
and the lavas ejected should be consequently acid, as in fact they are, 
while the later eruptions, reaching down to deeper and heavier zones 
of the magma, would bring up such basic lavas as basalt. Certainly 
the general similarity of the volcanic rocks all over the globe would 
appear to prove that there must be considerable uniformity of com- 
position in the zones of intensely hot material from which volcanic 
rocks are derived, and the general order of succession in the appear- 
ance of lavas, shows that some arrangement in relation to density 
probably exists within the crust.’ 
Many difficulties, however, remain yet to be explained before our 
knowledge of volcanic action can be regarded as more than rudi- 
mentary. Jor example why should two adjoining vents, like Mauna 
Loa and Kilauea, have their lava column at such widely different 
levels as to show that there can be no real connection between them ? 
Why should two neighbouring vents sometimes eject, the one acid, 
the other basic lavas? Why should even the same vent occasionally 
exhibit an alternation of acid and basic eruptions? To these and 
other questions in the mechanism of volcanoes no satisfactory answers. 
have yet been given. In Book IV., Part VIL, a description is given 
of the part voleanic rocks have played in building up what we see 
of the earth’s crust, and the student will there find other illustrations 
of facts and deductions which have been given in the previous pages. 
Section II.—Earthquakes.° 
The term Earthquake denotes any natural subterranean con- 
cussion, varying from such slight tremors as to be hardly perceptible 
' See 8. von Waltershausen, Sicilien und Island, p. 416. Reyer, Beitrag zur Physik 
dey Eruptionen, iii. 
? In the memoir by Captain Dutton, cited in a previous note, the hypothesis is main- 
tained that the order of appearance of the lavas is determined by their relative density 
and fusibility, the most basic and heaviest, though most easily fused, requiring the 
highest temperature to diminish their density to such an exteut as to permit them to be 
erupted. 
* On the phenomena of earthquikes consult Mallet, Brit, Assoc, 1847, part ii. p. 30; 
