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brown; the biotite is always brown and strongly pleochroic. Often these two minerals are clustered together irregularly or in parallel growths. They have generally a fairly strong tendency to idiomorphism, but may sometimes enclose plagioclase felspar in ophitic manner. Both of them decompose to See also:

CHLORITE

chlorite, See also:

EPIDOTE

epidote and See also:

CARBONATES

carbonates. The plagioclase felspar, which may See also:

FORM (Lat. forma)

form more than one-See also:

HALF

half of the rock, is See also:

ANDESINE

andesine or See also:

OLIGOCLASE

oligoclase; See also:

SIMPLE

simple crystals are rare, the See also:

MAJORITY (Fr. majorite; Med. Lat. majoritas; Lat. major, greater)

majority being complex growths with centres of felspar rich in See also:

lime, while in the See also:

EXTERNAL

external zones the proportion of soda felspar increases greatly. The inner portions have often well-defined, but very irregular, boundaries, and are sometimes sponge-like, with the cayities filled up with a later, more See also:

ACID (from the Lat. root ac-, sharp; acere, to be sour)

acid, See also:

DEPOSIT (Lat. depositurn, from deponere, to lay down, to put in the care of)

deposit. This seems to indicate that growth has taken See also:

PLACE (through Fr. from Lat. platea, street; Gr. IrAar6s, wide)

place in stages, alternating with periods when the crystallized felspar was eroded or partly dissolved. The orthoclase sometimes forms irregular plates enclosing individuals of plagioclase.

Quartz occurs both in irregular simple grains and as micropegmatite. Occasionally See also:

PALE (through Fr. pal, from Lat. palms, a stake, for paglus, from the stem pag- of pangere, to fix; " pole " is from the same original source)

pale green See also:

PYROXENE

pyroxene is visible in the centre of crystals of dark green hornblende. The See also:

ACCESSORY

accessory minerals See also:

APATITE

apatite, See also:

MAGNETITE

magnetite and See also:

ZIRCON

zircon are always See also:

PRESENT

present, and very See also:

COMMON

common also are orthite in See also:

COFFEE (Fr: cafe, Ger. Kaffee)

coffee-coloured zonal prisms practically always encircled by yellow epidote, and reddish-brown crystals of See also:

SPHENE

sphene, simple or twinned. In external See also:

APPEARANCE (from Lat. apparere, to appear)

appearance the tonalites are very like the granites but usually darker in See also:

COLOUR (Lat. color, connected with celare, to hide, the root meaning, therefore, being that of a covering)

colour. Tonalite-porphyrites often accompany them, having the same See also:

composition but with phenocrysts of felspar, quartz, hornblende and biotite in a See also:

FINE

fine-grained ground-See also:

mass. See also:

VEINS

Veins and threads of fine See also:

grey rock, mainly composed of quartz and felspar, often intersect tonalite-masses and have been called tonalite-aplites, seeing that they See also:

bear the same relations to aplites as the aplites do to the granites. They contain more soda-lime felspar than the normal aplites. Towards their margins the larger alpine masses of tonalite often assume banded or gneissic facies, due apparently to See also:

MOVEMENT

movement during intrusion.

End of Article: TONALITE

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