matic, very perfect; in other directions imperfect. Fracture uneven. H. 5-6. S. G. 2.9-34. Colour passing from white through various shades of green and brown to black. Streak either colourless or lighter than the colour of the mineral. Lustre vitreous, on cleavage surfaces mother-ofpearl; the fibrous varieties silky. All degrees of transparency to the opaque. Cp. variable. We may give as a normal formula, R3Si2+RSi, in which R = Mg, Ċa, and Fe, and Si is sometimes partially replaced by Äl. Bp. these minerals usually fuse, with intumescence, to a grey, greenish, or black glass, and the more readily the more iron they contain. The varieties richest in iron are partially decomposed by muriatic acid; other varieties are little affected by that acid.

(a) Tremolite (Grammatite, Calamite). — Of light colour, semi-translucent. Iron not an essential ingredient. Cp.= Mg3Si2+ CaSi=60Si+27Mg+12Ċa.

Usually imbedded in granular limestones and dolomites, in the form of long columnar crystals, or long stalklike or fibrous masses.

(b) Actinolite (Strahlstein, Glassy Actinolite).-Colour green. Cp. like tremolite. Occurs as an accessory in talcschist, chlorite-schist, &c. also as an independent rock (actinolite-schist).

(c) Hornblende (proper). Colour dark-green or black;

opaque. Cp. rich in iron and alumina. Forms an independent rock of itself, or occurs as an essential constituent of many compound rocks (syenite, diorite, many kinds of gneiss and porphyry). Occurs in the form of very perfect brownish-black crystals, imbedded in basaltic and trachytic rocks. A variety of the mineral is termed gamsigradite, and forms an essential constituent of the rock timazite.

C

(d) Uralite has the same cleavage, structure, and composition as hornblende, but the exterior form of augite. Crystals

of this mineral occur in many greenstones (uraliteporphyries). (Predazzo.)

(e) Asbestus and amianthus are fibrous varieties of tremolite and actinolite. In the variety known as Mountain leather the fibres are closely interlaced, or woven like felt. These minerals fill cavities and clefts in limestone and serpentine.

(f) Nephrite and Jade may be here added. They consist of a compact white or light-green translucent mass, with splintery fracture. Cp. very variable, sometimes that of tremolite. It is not a rigidly-defined mineral; forms independent layers as deposits between talcose rocks (in Turkey, New Zealand, &c.).

16. Pyroxene (Augite).-Monoclinic. In crystals imbedded or attached, or in stalklike, scaly, or granular masses. Cleavage prismatic, but usually less perfect than hornblende. Fracture conchoidal to uneven. H.=5—6. S.G. 3.2-3.5. Rarely colourless. Colour usually grey, green, or black. Lustre vitreous, sometimes mother-of-pearl. All degrees of transparency. Cp.=R3Si2, but very variable. Si partly replaced by Äl; R=Ca, Mg, Fe, Mn. Bp. the pyroxenes fuse (some quietly, others with some effervescence) to a white, grey, green, or black glass. Usually they are with difficulty reducible by microcosmic salt; those that contain Äl almost not at all. Almost all exhibit the reaction for iron, the white and light-coloured varieties manganese. Imperfectly decomposed by acids. The following mineralogical varieties are distinguished:

(a) Diopside.-Light-coloured, transparent and translucent varieties, and

(b) Salite.-Green, translucent only at the edges; usually

foliated. This and the last are without much geological importance. A sahlite, termed malakolite, is however found separately imbedded in the granular limestone. (c) Augite.-Green to black, opaque. Occurs as an essential ingredient in basalt, dolerite, diabase, and many lavas. Frequently in the form of perfect crystals porphyritically imbedded. Also found in meteoric stones.

(d) Omphazite.-Grass-green, always accompanied by garnet, and together with it forming eklogite.

(e) Hypersthene (Paulite).-Reddish-brown, greenish-black, or black, with metallic mother-of-pearl lustre on the faces of most perfect cleavage, and sometimes a change of colours showing a copper-red tinge. Lustre otherwise vitreous or resinous. In thin lamellæ translucent. Cp. very poor in lime, rich in iron and manganese. Hypersthene is an essential constituent of the rock hypersthenite (Penig in Saxony, Isle of Skye, Southern Tyrol). Otherwise it is usually an accessory, and is especially frequent in gabbro.

Appendix to Pyroxene.

Diallage (Smaragdite), which is an essential constituent of many gabbro rocks, is only a peculiar variety of pyroxene or hornblende, or perhaps a mixture of both.

The following are hydrous products of the decomposition of pyroxene :

Schillerspar. An essential constituent of schiller rock (Baste, in the Harz), accessory in serpentine.

Palagonite. The principal ingredient of the tufa of that name (Sicily, Nassau).

Green Earth.-Frequent in vesicular cavities of amygdaloids and in basaltic tufas.

The distinction between hornblende and pyroxene tremely important lithologically, but is often attende considerable difficulty. Some assistance may be deriv the following remarks:

(a) As to Crystallographic Differences.-Only recognis cases of granular texture, where the crystals are to perfect. One of the most essential and bestdifferences consists in the different angles cleavage prisms of the two minerals (and th usually identical with the angles of the exteri of the crystal.) In hornblende the larger angle prism is 124° 30' (giving a complement of 55 in pyroxene the angles are 87° 5' to 92° 55'. (3) Differences of Paragenesis.-Rocks containing free felspars rich in silica (such as orthoclase and or potash-mica as essential constituents, likewise contain augitic minerals, but if th occur, they are almost invariably hornblende, pyroxene. In pyroxenic rocks, quartz espec very rarely found, and if present is only ac (eklogite, hypersthenite). On the other hand dorite and magnesian micas are very frequent rocks, though not exclusively there found.

Pistacite and pyrites are more frequent acc in hornblendic than pyroxenic rocks. The pis found adhering to the surfaces of clefts, or in cavities, and would appear in most cases to be duct of the decomposition of amphibolite.

Leucite and olivine are characteristic as ad minerals in pyroxenic rocks.

As a very general rule, we may characteris blende as the constituent of the plutonic, pyro

that of the volcanic igneous rocks. Nevertheless, sometimes both are found together in the same rock (basalt, omphacite, trachyte of Etna). In this latter case the pyroxene is the older formation of the two, i.e. it has cooled and become solid more rapidly than the hornblende.

(y) The chemical differences between pyroxene and hornblende are not marked. It would appear as if one or the other might have resulted from the same identical mass according to the conditions under which it cooled and solidified. The origin both of hornblende and of pyroxene may be of various kinds.

The possibility of their formation by wet process during the development or the transmutation of a rock's mass has been proved by Daubrée, who actually produced diopside by subjecting glass to the influence of the thermal waters of Plombières.

Many of the crystals which are found disseminated in limestone rocks would appear to be the result of metamorphosis (Pargas, Tyrol).

Again, both these minerals may be products of sublimation (Elie de Beaumont, Sacchi), or they may be simple products of igneous action, since we find in the slags of smelting furnaces products of precisely similar form and composition.

17. Spodumene (Triphane).-Monoclinic, isomorphous with pyroxene; crystallised or massive in broad fibrous or scaly masses. Cleavage orthodiagonal and prismatic. Fracture uneven. H.-6.5-7 S.G.=3.1-3.2. Lustre vitreous, with mother-of-pearl lustre on the cleavage surfaces. Colour greenish-grey to apple-green. Translucent, but frequently only at the edges. Cp.=Li3Si2+4AlSi2 frequently with some Na, K, or Ċa. Bp. intumescent; colours the flame red, but