50 Onthe Minerals of Trap and the allied Rocks. 
spect, the essential constituents of trap and basalt are like those 
of granite and syenite. But in the second group, consisting of 
thie. minerals occurring in cavities or seams, all contain water 
except pectolite, quartz, calc spar and spathic iron ; and the last 
three are known to be always deposited in an guhydroys state 
from aqueous solutions. 
We proceed to give a few brief hints with regard to the first 
group, intending only to glance at this branch of the subject, and 
then take up more at length the group of adventitious minerals. 
Essential constituents of modern Plutonic rocks.—lIt is ob- 
vious that modern igneous rocks, although in some cases derived 
from the original material of the globe, have proceeded to a great 
extent from a simple fusion of rocks previously existing, and 
especially of the older igneous rocks. In accordance with this 
view, we may with reason infer that the trachytes and porphy- 
ries, which consist essentially of feldspar, have proceeded in 
many instances at least, from feldspathic granites; the basalts 
and trap from syenites, hornblende or augitic rocks. 
A theory proposed by Von Buch supposes that the feldspathic 
rocks, as they are of less specific gravity, are from the earliest 
eruptions, or the more superficial fusings, while the heavier ba- 
lt has come from greater depths. Darwin thus accounts for 
the granites of the surface being intersected by basaltic dykes ; 
the latter having originated from a deeper source, where their 
constituents took their place at some former period from their 
superior gravity. It virtually places hornblende rocks below 
feldspathic granites in the interior structure of our globe. The 
hypothesis is ingenious and demands consideration ; but it may 
not be time to give it our full confidence. 
But supposing these more modern rocks to have been derived 
from the more ancient granitic—what has become of the quartz 
and mica which occur so abundantly in the latter, while they 
are so uncommon in the former? By what changes have they 
disappeared ? 
In the fusion produced by internal fires, the elements are free 
to move and enter into any combinations that may be favored 
by their affinities. If silica, alumina, magnesia, lime, iron, the 
alkalies, potash and soda, were fused together—and these are the 
actual constituents of basalt—what result might we expect? 
From known facts, we should conclude that. the silica would 
