TONALITE  , in

petrology, a rock of the diorite class, first described from Monte Adamello near Tonale in the Eastern Alps . It may be described as a quartz-diorite containing biotite and hornblende in nearly equal proportions . The principal felspar is plagioclase, but orthoclase occurs also, usually in small amount . Those varieties which are rich in orthoclase, in addition to plagioclase, have been called quartz-monzonites or adamellites, but a better term is grano-diorite, which has been very generally adopted in America for rocks which are intermediate in character between the granites and the diorites . The hornblende of the diorites is green, sometimes with a tinge of 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 chlorite, epidote and carbonates . The plagioclase felspar, which may form more than one-half of the rock, is andesine or oligoclase; simple crystals are rare, the majority being complex growths with centres of felspar rich in lime, while in the 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 acid, deposit . This seems to indicate that growth has taken 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

pale green pyroxene is visible in the centre of crystals of dark green hornblende . The accessory minerals apatite, magnetite and zircon are always present, and very common also are orthite in coffee-coloured zonal prisms practically always encircled by yellow epidote, and reddish-brown crystals of sphene, simple or twinned . In external appearance the tonalites are very like the granites but usually darker in colour . Tonalite-porphyrites often accompany them, having the same composition but with phenocrysts of felspar, quartz, hornblende and biotite in a fine-grained ground-mass . Veins and threads of fine grey rock, mainly composed of quartz and felspar, often intersect tonalite-masses and have been called tonalite-aplites, seeing that they 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 movement during intrusion .