‘On the Minerals of Trap and the allied Rocks. 51 
combine with the different bases, and these simple silicates 
would unite into more complex compounds. The silicates of 
alumina and the alkalies or lime, form thus one set of compounds, 
the feldspars; the silicates of magnesia and the isomorphous 
bases, iron and lime, another set, to which belong augite, horn- 
blende and chrysolite; and if much iron is present, we might 
have with the lime and alumina, the mineral epidote. The ex- 
periments of Berthier, Mitscherlich, and Rose, and the facts ob- 
served among furnace slags, confirm what is here stated. 
But not to go back to a resolution of the fused minerals into 
their elements, we may consider for a moment what changes 
the minerals themselves might more directly undergo in the pro- 
cess of fusion. 
Much of the mica in granite, differs from febdgpnd in contain- 
ing half the amount of silica in proportion to the bases—the 
bases in each being alumina and potash or soda. The change 
then in the conversion of the mica into feldspar, would require 
an addition of silica, which might be derived from the free quartz 
of granite. Other varieties of mica contain magnesia, which 
would go towards the formation of some mineral of the mag- 
nesian series. It is possible that trachytes and porphyry have 
thus been made from granite; but trap rocks could not have 
been so derived, as they contain from 10 to 25 per cent. less of 
silica. 
Again, hornblende and augite are so nearly related, that 
they have been considered by Rose the same mineral, the differ- 
ent circumstances attending the cooling giving rise to the few 
peculiarities presented. ‘There can be no difficulty therefore in 
deriving augite by fusion from hornblende rocks. This moreover 
has been actually confirmed by experiment. 
Augite by giving up half of its silica, and receiving additional 
magnesia in place of its lime, is reduced to chrysolite.* The 
Gehlenite, nepheline, anorthite and meionite of Vesuvius, contain 
like scapolite, only 40 to 45 per cent. of silica and a large pro- 
portion of lime, and it is no improbable supposition, judging from 
the small amount of silica, and from the lime present, that scapo- 
lite rock, or rather Jimestones containing scapolite, may have con- 
tributed in part towards the men of that region. The ejections 
“ The Pst of augite is R3 Si2; that of chrysolite, R8 Si. 
