ave been well described
by Captain Hart, and subsequently by Captain Robertson, who has paid a
visit to that region, and made sketches of them, which he has kindly
placed at my disposal. From one of these the annexed view has been
selected. These conical hills occur to the westward of the Hara
mountains and the river Hubb. (See Map, p. 460.) One of the cones is 400
feet high, composed of light-colored earth, and having at its summit a
crater thirty yards in diameter. The liquid mud which fills the crater
is continually disturbed by air-bubbles, and here and there is cast up
in small jets.[618]

_Mineral composition of volcanic products._--The mineral called felspar
forms in general more than half of the mass of modern lavas. When it is
in great excess, lavas are called trachytic: they consist generally of a
base of compact felspar, in which crystals of glassy felspar are
disseminated.[619] When augite (or pyroxene) predominates, lavas are
termed basaltic. They contain about 50 per cent. of silica, or much less
than the trachytes, in which there is usually about 75 per cent. of that
mineral. They also contain about 11 per cent. of protoxide of iron, and
as much of lime, both of which are wanting, or only in insignificant
quantities in the trachytic rocks.[620] But lavas occur of an
intermediate composition between the trachytic and basaltic, which from
their color have been called graystones. The abundance of quartz,
forming distinct crystals or concretions, characterizes the granitic and
other ancient rocks, now generally considered by geologists as of
igneous origin; whereas that mineral is rarely exhibited in a separate
form in recent lavas, although silica enters so largely into their
composition. Hornblende, so common in hypogene rocks, or those commonly
called "primary," is rare in modern lava; nor does it enter largely into
rocks of any age in which augite abounds. It should, however, be stated,
that the experiments of Mr. Gustav Rose have made it very questionable,
whether the minerals called hornblende and augite can be separated as
distinct species, as their different varieties seem to pass into each
other, whether we consider the characters derived from their angles of
crystallization, their chemical composition, or their specific gravity.
The difference in form of the two substances may be explained by the
different circumstances under which they have been produced, the form
of hornblende being the result of