ba 
ca a ee 
Origin of the Constitution of Tgneotis Rocks. 349 
on the earth, proceeds principally from the: vapors of water and 
sulphur, which are constantly rising through them, inflating them 
More and more as they ascend, and finally escaping in bubbles 
at the surface. Now the feldspar being the less fusible part of 
the lavas, would thicken somewhat, wherever the temperature 
became too low for complete fusion; the more liquid portion 
would then ascend most easily, being carried along by the infla- 
ting vapors, and much of the feldspar would thus be left behind, 
and it might be in a nearly pure state. 'The centre of the vol- 
' cano-under this action, becomes necessarily feldspathic. The 
summit might therefore eject either basaltic or feldspathic rocks 
from the material of the vent; though when the action was 
violent and deep, it would eject feldspathic rocks alone. , 
“At the same time the basaltic lavas, descending laterally in 
this system of circulation along the sides of the great central con- 
duit, may pass out as flank eruptions through fissures. Besides, 
there will also be basaltic ejections from sources of lavas at a dis- 
tance from the central conduit, where they have not been sub- 
jected to the separating process described ; and this may be the 
More common source. 
Mountains with a feldspathic centre, and basaltic layers form 
ing the circumference, are therefore quite intelligible without 
Supposing the feldspar to have been first thrown up, or appealing 
toa different system of fissures for their origin, and the examples 
Which the moon presents, are more extensive than is necessary 
to explain the widest facts on the earth.* phic 
In these remarks we have spoken of the lavas as consisting 
mainly of feldspar and augite, their more common constitution ; 
but we use the terms in a general sense, understanding by feld- 
Spat one or another of the feldspar family of minerals, and by 
2 SRM err ofr or ae er cree 
% 
sequently that there would have been scoria and cinders 
lions? or may we believe it probable, that the paste was so 
Would not make its ‘way up and escape as vapor? Is this last supposition borne 
Sut by any existing example of subaérial volcanic action? 
