164 © J. D. Dana—Results of the Earth's Contraction. 
The former include the rocks containing as essential constit- 
uents one or more of the iron-bearing minerals, augite, 
blende and chrysolite, and often also magnetite ; and are divided 
into two groups, the doleritic, containing pyroxene, and the 
syenitic containing hornblende in place of p : 
The latter comprise those mostly (seldom wholly) free from 
these iron-bearing minerals, as the trachytic and granitic kinds. 
(d.) The presence of quartz among the chief constituent min- 
erals of the true crust is not certain. Of the above-mentioned 
rocks, the basic iron-bearing (or doleritic) kinds are far the most 
abundant among acknowledged igneous rocks; and this fact 
seems to indicate that quartz or free silica was not abundant in 
the original liquid rock of the globe. Its absence, which Mr. Hunt 
urges, is seemingly nl grant by the fact that it is present in so 
many trachytes, as well as in syenite and granite, and the related 
rocks. But Hunt is right in holding that in general granite 
and syenite (the quartz-bearing syenite) are undoubtedly meta- 
morphic rocks where not vein-formations, as I know from the 
study of many examples of them in New England; and the 
veins are results of infiltration through heated moisture from 
the rocks adjoining some part of the opened fissures they fi 
These rocks, although common, present therefore no positive 
testimony on the side of the presence of quartz. Mr. Hunt urges, 
in support of his opinion, the experiment of Rose, in whic 
fused quartz on cooling had the low density and other characters 
of the form of silica called opal, and not those of quartz. But 
the evidence is inconclusive, since a laboratory experiment can- 
not inform us what would be the condition of silica on cooling 
from fusion, provided the process of solidification took some 
wholly, in combination, and that the chief feldspars present 
were the lime-and-soda species, labradorite and oligoclase. 
Granite and syenite—common rocks of Archean terranes— 
