Geologists were, at one time, almost unanimous in the opinion that all serpentine is of eruptive origin; but now it is conceded by the great majority to be in some cases a sedimentary rock. It is found interstratified with gneiss, limestone, all the schists, and many other stratified rocks. When occupying the position of an eruptive it is never an original rock; but has been formed by the alteration, _in situ_, of some basic anhydrous rock, most commonly olivine basalt. Greensand.—This rock (specimen 27) consists chiefly of the mineral glauconite, mingled usually with more or less sand, clay, or calcareous matter. It is usually very friable, or in an entirely unconsolidated state. It is most abundant in the newer geological formations, especially the Cretaceous and Tertiary; and is, perhaps, the only one of the stratified silicate rocks now forming on an extensive scale in the ocean. Its value as a fertilizer, for which purpose it is extensively employed, is due to the potash that it contains. Following is a systematic summary of the mineralogical composition of the rocks of this great division of silicates; and this, combined with the classification on page 69, presents in a condensed form all the more important facts contained in the preceding descriptions. Only the more constant and normal constituents of the species are enumerated in each case:— ====================+==================================== Names of Species. | Constituent Minerals. --------------------+------------------------------------ ⎧| Orthoclase and Quartz. Gneiss ⎨| Orthoclase, Quartz, and Mica. ⎩| Orthoclase, Quartz, and Hornblende. --------------------+------------------------------------ ⎧| Orthoclase. Syenite ⎨| Orthoclase and Hornblende. ⎩| Orthoclase and Mica. --------------------+------------------------------------ ⎧| Plagioclase (chiefly Oligoclase). Diorite ⎨| Plagioclase and Hornblende. ⎩| Plagioclase and Mica. --------------------+------------------------------------ ⎧| Plagioclase (chiefly Labradorite). Norite ⎨| Plagioclase and Augite (Diallage). ⎩| Plagioclase and Mica. --------------------+------------------------------------ ⎧| Mica. Mica Schist ⎨| Mica and Quartz. ⎩| Mica and Kaolin. --------------------+------------------------------------ Hornblende Schist | Hornblende and Quartz. --------------------+------------------------------------ Amphibolite | Hornblende. Pyroxenite | Pyroxene. --------------------+------------------------------------ Talc Schist | Talc.