The first sub-group, called here "trachytic andesites," corresponds to diorites rich in soda. The second, the basaltic andesites," corresponds to the pyroxene-diorites with basic felspars, and thus to the bulk of "gabbros without olivine."

Sub-group 1.-Trachytic Andesites. Structure-Like trachyte. Commonly porphyritic. Constituents-1, Plagioclase (commonly Oligoclase); 2, Soda-Augite, Hornblende, or Mica. Sometimes Rhombic Pyroxene. Lithoidal to glassy groundmass.

I. The marked feature of the andesites is the absence of orthoclase; in this sub-group the striated oligoclases are abundant and the ferro-magnesian constituents are less important. The groundmass is characteristically trachytic; colour on the whole darker than in trachyte. Spherulitic and other structures characteristic of the more glassy rocks are rare.

The much altered and older examples (many of the "Porphyrites") are typically brown-red and almost earthy in appearance. Search should be made in the field for the least altered. portions of the mass.

Specific Gravity.—About 2.75.

Typical Analyses. -Poor in magnesia, and fairly rich in alkalies.

A. Hornblende-Andesite. Wolkenburg, Siebengebirge. Bischof, Lehrb..

d. Geol., 1 Aufl., Bd. ii., p. 2181.

B. Hornblende-Andesite with Augite. Puy de Louchadière, Auvergne. Von Lasaulx, Neues Jahrb. für Min., 1869, p. 708.

II. The broad sections of felspar that characterise this typeof andesite are often as fresh and clear as sanidine, but show beautiful twin-lamellation. The glassy groundmass has com

monly penetrated them and worked far into their interiors, the

corrosion spreading easily along the planes of composition of the twins. The extent to which these plagioclases have yielded to the attack of the magma is a feature of great interest; and the external matrix has often become dull by the development of crystallites, while the P intruded portions have preserved a purely glassy character.

P Fig. 35. Hornblende- Andesite (trachytic type). Summit of Beinn Nevis, Scotland. x 7. h, Brown hornblende. P, Plagioclase, often much corroded by the glass around. Fluidal hemicrystalline ground

The hornblendes or micas, again, suffer by the development of an black margin, opaque and sometimes remain as black granular pseudomorphs.

Rich brown biotite is again and again associated in these rocks with hornblende. The typical pyroxene is very pale green and is probably soda-augite; and rhombic forms, generally poor in iron, may appear.

mass.

The groundmass is brownish and trachytic in appearance. The glass, where traceable, is pale and almost colourless. Evidences of flow are less frequent than in the trachytes.

The "Porphyrites" (altered andesites) of this sub-group show typically a brown earthy matrix, often with green pseudomorphs after biotite. The hornblende and pyroxene have commonly become completely decomposed, leaving colourless areas bounded and traversed by strong opaque bands, which are formed by the iron oxides separated out along the cracks and on the margins of the original crystals. The glass of the groundmass, and that intruded into the felspars, can sometimes be traced as yellow areas occupied by decomposition-products, which resemble serpentine between crossed nicols.

Sub-group 2.-Basaltic Andesites. Typically Pyroxene-Andesites. "Basalt without olivine" comes here, when there is about 50 per cent or more of silica. Structure-Lithoidal; sometimes with glassy interspaces between the crystals. Constituents-1, Plagioclase (Oligoclase or, probably more often, Labradorite); 2, Augite or Rhombic Pyroxene; more rarely Hornblende and Mica. Magnetite is conspicuous. Lithoidal to glassy groundmass.

I. In appearance these rocks are darker and compacter than

those of the preceding sub-group, and approach the basalts in texture, becoming even black and notably heavy. The rock tends to break conchoidally, and a spheroidal structure in the mass is not uncommon, which is developed, with onion-like effect, by weathering. The porphyritic crystals of plagioclase are often accompanied by well developed pyroxene, the stout black prisms of which stand out on the surface amid the deep brown groundmass.

The mass of the rock appears microcrystalline to the eye and lens, the small rod-shaped felspar prisms being often discernible. It is scratched by the knife, leaving a light streak. When much glass is present, dark areas appear, with a quartz-like aspect and conchoidal fracture, between the crystals, and the whole rock may have a speckled vitreous lustre when turned about in the hand. Spherulites, banding, &c., are rare; and scoriaceous rather than pumiceous structure accompanies the examples gathered from lava-streams.

The much altered types (part of "Porphyrite" and "Diabase ") are commonly reddish, like those derived from the trachytic andesites; or compact black, like many of the rocks styled by Brongniart" Melaphyre," * a number of which must come into this sub-group.

Specific Gravity.-About 2.75 to 2.9.

Typical Analyses. - Richer in lime and magnesia and poorer in alkalies than preceding sub-group.

A. Augite-Andesite.

Tunguragua, Andes. Artopé, quoted by Roth, Beiträge zur Petrogr., 1873, p. xlvi.t

B. Hypersthene-Augite-Andesite, Buffalo Peaks, Colorado. Hillebrand, Bull. U.S. Geol. Survey, No. 1, p. 26.

C. Augite-Andesite ("Basalt without olivine"). Ann. d. Chem. u. Pharm., 1848, p. 22.

Hals, Iceland. Genth,

Fe O

Classification minéral. des Roches Mélangées, 1813, p. 40.

The glassy augite-andesite of Eskdale, Dumfries. has a closely similar

composition. See Teall, Petrogr., p. 196.

Compare description of olivine-basalts, p. 257.

II. In sections the prominence of pyroxene, whether pale or strongly yellow-brown and purple-brown, and the comparative lack of hornblende and mica, strike the eye at once. The latter minerals are, in fact, typically absent.

Augite occurs porphyritically, and has developed abundantly in the groundmass in the more basic types ("basalts without olivine"), occurring there as grain-like crystals between rodshaped felspars; the rock passes, by exclusion of the interstitial cryptocrystalline matter, into typical "dolerite without olivine." Enstatite or hypersthene is common (fig. 36, 4).

The plagioclase, when porphyritic, is freely corroded, preserving a general prismatic outline, though the interior may be largely replaced by a maze-like structure of brown glass. As above remarked, a mesh of rod-shaped plagioclases developes in the groundmass in the most basic types.

In some varieties of andesite from near Tetschen in Bohemia

X

Fig. 36.-A, Pyroxene-Andesite (basaltic type). Kremnitz, Hungary. 14. p, Plagioclase. r.p, Rhombic pyroxene (enstatite). Cubes of magnetite occur. Dark hemicrystalline groundmass. B, Glassy Pyroxene-Andesite (basaltic type). Dyke, Eskdale, Dumfries. × 40. a, Granular augite, often set with radiating microlites from the glassy groundmass. p, Plagioclase in various stages of growth, often with characteristic bifurcating and incomplete terminations. Magnetite occurs. This rock exhibits the clear brown interstitial glass typical of many continental augite-andesites and "porphyrites."

the felspar is mainly in the cryptocrystalline groundmass, in which abundant microlites of brown hornblende have developed.

The porphyritic crystals are, on the other hand, large augites, with very pronounced idomorphic characters in section. This rock will serve to show how removed the basaltic andesites may be from the trachytic type.

The groundmass is characteristically brown, with at times skeleton-crystals (cross-like forms) of magnetite. When completely glassy, it is a warm transparent brown (fig. 36, B), in which the well-defined crystals of the final consolidation lie.

When the groundmass appears filling the interstices of the felspar mesh, it is described by Rosenbusch as "intersertal." Were it now to become converted into large crystals, it would often result in an ophitic structure, since its composition must often be near that of a pyroxene, the felspathic matter having been withdrawn from it. In the same rock-section the structure of a basaltic andesite with a felspar mesh may be seen in one part, and that of ophitic dolerite in another (compare fig. 39).

The "Porphyrites" of this sub-group show a yellowish substance in the place of any original glass. The rhombic pyroxenes are decomposed to green fibrous forms; the augites are often replaced by chlorite, and the felspars in large part by calcite. Specks of calcite may also appear throughout the groundmass.

Varieties of Andesite.-Beyond the above broad divisions of the andesites, we may expect the following varieties:

NEPHELINE-ANDESITE (NEPHELINE-TEPHRITE). The "Tephrites" are a plagioclase-series parallel to the phonolites, and commonly containing soda-augite. The name is unfortunate, since the old "Téphrines" are rarely "tephrites" in the restricted sense of Rosenbusch, being mostly rough grey andesites. "Basanite" of Rosenbusch is a tephrite with olivine; such rocks will be classed here as varieties of olivine-basalt. The "basanite" of Brongniart (1827) was merely a porphyritic basalt. "tephrite" is practically an andesite with part of the felspar replaced by a felspathoid (p. 247). The nepheline-andesites seem rarer than the nepheline-trachytes. The silica sinks to about 50 per cent.

A

LEUCITE-ANDESITE (LEUCITE-TEPHRITE). The leucites are often conspicuous on the surface of the rock, as in the fine example from Cività Castellana near Viterbo.

NOSEAN- or HAÜYNE-ANDESITE (NOSEAN-TEPHRITE).-Haüyne is more prevalent than nosean, doubtless owing to the presence of lime rather than soda in the molten rock. A very fine example is the so-called "Haüynophyre" of Melfi; some parts of this rock, with only 43 per cent. of silica, cannot fairly be ranged as andesite. Note to the Andesites.-PROPYLITE, used by von Richthofen for the oldest Tertiary andesites, has been revived by Rosenbusch for those forms in