tono_, from Gr. [Greek: barutonos], deep
sounding), a musical term for the male voice whose range lies between those
of the tenor and of the bass--a high bass rather than a low tenor; also the
name of an obsolete stringed instrument like the viola da Gamba, and of the
small Bb or C saxhorn.

BASALT, in petrology, one of the oldest rock names, supposed to be derived
from an Ethiopian word _basal_, signifying a stone which yields iron;
according to Pliny, the first basalts were obtained in Ethiopia. In current
usage the term includes a large variety of types of igneous rock belonging
to the basic subdivision, dark in colour weathering to brown, and
comparatively rich in magnesia and iron. Some basalts are in large measure
glassy (tachylites), and many are very fine grained and compact; but it is
more usual for them to exhibit porphyritic structure, showing larger
crystals of olivine, augite or felspar in a finely crystalline groundmass.
Olivine and augite are the commonest porphyritic minerals in basalts, the
former green or yellowish (and weathering to green or brown serpentine),
the latter pitch-black. Porphyritic plagioclase felspars, however, are also
very common, and may be one or two inches in length, though usually not
exceeding a quarter of an inch; when fresh they are dark grey with smooth
lustrous cleavage surfaces; when decomposed they become turbid, and assume
grey or greenish shades. Basaltic lavas are frequently spongy or pumiceous,
especially near their surfaces; and, in course of time, the steam cavities
become filled with secondary minerals such as calcite, chlorite and
zeolites. Another characteristic of this group of rocks is the perfection
with which many of them show prismatic or columnar jointing, a structure
often called "basaltic jointing."

The minerals of basaltic rocks have a fairly uniform character throughout
the whole group. In microscopic section the olivine is pale green or
colourless, and is very frequently more or less altered to serpentine. The
secondary mineral begins to form upon the surfaces and along the cracks of
the olivine, gradually producing a mesh-work in the interstices of which
small kernels of olivine remain; and when the process is completed the mesh
structure persists in the resulting pseudomorph, giving a clear indication
as to its history. The augite is mostly brown, often with a purplish tinge,
hardly at all dichroic, but frequently showing zonal or hour-glass
structure