Page:A Treatise on Geology, volume 2.djvu/98

D. Siliceous trachyte, when there appears to have been introduced a great quantity of silex into its composition.

Greystone.

A. Common, consisting of felspar, augite (or hornblende), and iron.

B. Leucitic greystone, when leucite supplants the felspar.

C. Melilitic greystone, when melilite is substituted for that mineral, &c.

Basalt.

A. Common basalt, composed of felspar, augite, and iron.

B. Leucitic, when leucite replaces the felspar.

C. Basalt, with olivine in lieu of felspar.

D. Basalt, with hauyne in lieu of felspar.

E. Ferruginous basalt, when iron is the predominant ingredient.

F. Augitic basalt, when augite or hornblende composes nearly the whole of the rock.

If our knowledge of the true composition of many of the old rocks of fusion were perfect, we might propose for them a scale of classification parallel to that which Mr. Scrope has given for volcanic rocks. Of such a scale the following would appear to be the elements:—

Division I.—Felspathic.

Rocks in which the characteristic and most abundant mineral, felspar, is not at all or but slightly mixed with hornblende, augite, or their conveners, hypersthene, diallage, &c.

Ancient.	Modern.
Granitic and most porphyritic rocks.	Trachytic rocks of Von Buch, Cordier, Scrope, &c.

Division II.—	$\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.$	Hornblende,	$\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}$	Felspathic.
		Augite, &c.

Rocks in which felspar is mixed in nearly equal proportion with hornblende or augite, or their conveners, hypersthene, diallage, &c.