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378 PACIFIC ISLANDS. 
hornblende which are common in trachytes; and when the feldspar 
also becomes crystalline, the two together produce the syenitic rock 
we have described, specimens of which were obtained at Tahiti. The 
same remarks will also apply to mica, another of the minerals occa- 
sionally occurring in trachytes, and met with by the writer in an ex- 
tinct volcano on the Sacramento, in Northern California. 
The same cause which makes the feldspar centre of the volcanic 
mountain, will also cause to be detained within the feldspar (perhaps 
at considerable depths) the excess of quartz not in combination; for this 
mineral, like chrysolite, is of very difficult fusibility. Hence trachytes 
are often quite silicious. Hence, too, all the elements of true granite 
may occur within a volcanic focus, and as the fires die out and soli- 
dification takes place with extreme slowness, granite itself might form, 
and any of the common granitic minerals whose elements are present 
in the requisite proportions. 
Thus we arrive at the sarne statement with which we commenced— 
that particular rocks have no necessary relation to time on our globe, 
except so far as time is connected with a difference in the earth’s tem- 
perature or climate, and also in oceanic or atmospheric pressure: for 
if the elements are at hand, it requires only different circumstances 
as regards pressure, heat, and slowness of cooling, to form any igneous 
rock the world contains. The granite-like fragments thrown out from 
Vesuvius, consisting of glassy feldspar, mica, hornblende, spinel and 
other minerals, illustrate these principles. ‘They show that there has 
been slow cooling for a long period in some part of this crater, pro- 
ducing crystalline rocks, fragments of which later action has ejected. 
The fact that granites on our globe sometimes form a collection of 
summits with hornblende rocks surrounding in part the region, sug- 
gests a strong analogy to the trachytes with a circumference of 
basalts ;* and temperature will account for the difference. The size 
of these granitic regions will not be made an objection after the views 
and facts which have been presented. This analogy, however, should 
be cautiously applied. 
The occurrence of hornblendic and basaltic dikes so commonly 
over the globe, and pertaining to all periods, accords with this system 
of igneous operations. For like the dikes of a volcano, they come 
from a region where the separating process has not been in action. 
Such dikes, however, may often be feldspathic, since feldspathic 
* See Remarks by the author in the American Journal of Science, xlv. 125. 
