350 J.D. Dana on the Volcanoes of the Moon. 
augite the remaining fusible material, whether ordinary augite 
(silicate of lime, magnesia and iron) or silicates of one or more of 
these bases or alumina in other combinations. 
_ There is some difficulty in applying this hypothesis to particu- 
lar cases, on account of our ignorance of the actual fusibilities of 
the materials of the lava, in the condition in which they are pla- 
ced; for we know that an infusible.mineral may be held in fu- 
sion under certain circumstances, or with certain mineral associa- 
tions, far below the temperature at which it fuses: or, previous 
to the commencement of cooling it may be in some other combi- 
nation. - 
_ We should salen that the process which separates a feldspar, 
pelt also separate any excess of the more infusible mineral 
quartz. This may not follow: still it is a remarkable fact that 
the quantity of quartz contained in trachyte is often in great ex- 
cess, as analyses have shown. But why is not the infusible min- 
eral chrysolite also detained? The fact appears to be, that it is — 
of subsequent formation. The small proportion of silica it con- 
tains implies a deficiency of this substance, while, as we have 
stated, in the feldspathic rocks there is often an excess. It may, 
therefore, under certain circumstances proceed from the basaltic 
material, for its elements are the same in different proportions. 
ee investigations may give us more light on this point. 
The general principle which we have above brought f 
is well illustrated in the fact that the scoria or surface glassed 
any vent, where it occurs, is the most fusible part of the lava, 
consisting in general of ferruginous or alkaline silicates, and con- 
taining no magnesia. On account of the diminished heat, this 
material alone remains sufficiently fluid to be inflated and borne 
up to the surface by the rising vapors: and this takes place in 
spite of superior gravity. 
We hence comprehend the rapid cooling which characterizes 
ejected lavas, for only a part of the material is in complete fusion. 
The actual nature of the cooled igneous rock may be more cor 
rectly understood, if we consider that the minerals present 
depend, not only on heat and pressure and the causes above alla 
ded to, but also-on rate of cooling. The effect of slow cooling 
is 1s exemplified i in the feldspathic centre of a volcanic mountain. 
mg wholly . enclosed by, rocks, the heat of fusion ea 
and owing to the pressure of its own superincumbent 
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