52 On the Minerals of Trap and the allied Rocks. 
of unaltered granular limestones, and many mineral species per- 
taining to such beds, strongly support this view ; and it is no 
less sustained by the fact, that in the Vesuvian hesalte, Labra-. 
dorite, which includes lime instead of the alkalies, replaces com- 
mon feldspar. The original feldspar seems to have given way 
to leucite and Labradorite. 
An important source of new combinations is found in the sea- 
water which gains access to the fires of voleanoes. ‘The decom- 
position which takes place eliminates muriatic acid, so often de- 
tected among volcanic vapors ; but the soda and other fixed con- 
stituents remain, to enter into combination with some of the 
ingredients in fusion. Is not this one source of the soda forming 
the soda feldspar, or albite, and of the muriatic acid and soda in 
sodalite? Phosphates have been long known to occur occasionally 
in voleanic rocks, and lately phosphoric acid has been proved to 
be generally common in small quantities. Sea-water is also a 
very probable source of this ingredient, as has been shown by © 
late analyses of the same by Dr. Jackson. 
These few hints are barely sufficient to indicate something of 
the interest that attaches to this field of investigation, which the 
future developments of science will probably open fully to view. 
We do not attempt to explain why in these modern fusings, mica 
‘should not have remained mica, and the quartz still free uncom- 
bined quartz. The facts prove some peculiarity of condition at- 
tending the formation of the granitic rocks. Of this condition 
we know nothing certain, and can only suggest the common 
supposition of a higher heat and slower, cooling, attending a 
greater pressure and different electrical conditions, and the same 
circumstances may have existed during the granites of different 
ages 
With these brief suggestions, I pass to the second division of 
' the subject before us. 
_ 2. Minerals occupying cavities and } laloidal t 
or basalt.—These minerals have been attributed 1 to a variety of 
sources, and even at the present time there are various opinions re- 
* Using R for the bases and Si for silica, the formula of leucite is R sid; that of 
common feldspar, R Si2 ; that of Labradorite, R Si. From this, it appears that 
feldspar may be reduced to leucite by giving up one third of its silica, the bases 
being the same in the two; and with this excess and other silica combining with 
the lime at hand, Labendosite might be formed. 
