366 Prof. J. D. Dana on some Results of 



tuents one or more of the iron-bearing minerals (augite, horn- 

 blende, and chrysolite, and often also magnetite), and are divided 

 into two groups — the doleritic, containing pyroxene, and the sye- 

 nitic, containing hornblende in place of pyroxene. 



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 mine- 

 rals 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 opposed 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 

 metamorphic 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 fill. 

 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 

 which 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 cannot inform us what would be the condition of 

 silica on cooling from fusion, provided the process of solidifica- 

 tion took some millions of years. But Rose's experiment does 

 seem to settle the question as regards all quartz veins, since, if 

 of igneous origin, their little width would have ensured compa- 

 ratively rapid cooling ; and it thus sustains the evidence in favour 

 of the view that such veins, like most mineral veins, were filled 

 through the aid of heated moisture. Professor Hunt's argu- 

 ment from the probable condition of the material of the liquid 

 sphere when about to solidify at surface — or the fact that the 

 lime and magnesia now in our limestones and waters, and the 

 soda in our waters, must have been mainly in the condition of 

 silicates, and that therefore the free silica would thus be 

 in combination — is of great weight ; and, considering the vast 

 amount of limestone in the earth's formations, it favours the 

 view derived from the prevailing doleritic character of igneous 

 rocks, that silica was mostly, if not wholly, in combina- 

 tion, and that the chief felspars present were the lime-and- 

 soda species, Labradorite and oligoclase. Granite and syenite 

 (common rocks of Archaean terrains) are just the rocks that are 



