l pyroxenes. Frequently the
material is interlaminated with a rhombic pyroxene (bronzite) or with an
amphibole (smaragdite or uralite), the latter being an alteration
product of the diallage.

Diallage is usually greyish-green or dark green, sometimes brown, in
colour, and has a pearly to metallic lustre or schiller on the laminated
surfaces. The hardness is 4, and the specific gravity 3.2 to 3.35. It
does not occur in distinct crystals with definite outlines, but only as
lamellar masses in deep-seated igneous rocks, principally gabbro, of
which it is an essential constituent. It occurs also in some peridotites
and serpentines, and rarely in volcanic rocks (basalt) and crystalline
schists. Masses of considerable size are found in the coarse-grained
gabbros of the Island of Skye, Le Prese near Bornio in Valtellina,
Lombardy, Prato near Florence, and many other localities.

The name diallage, from diallage, "difference," in allusion to the
dissimilar cleavages and planes of fracture, as originally applied by R.
J. Hauey in 1801, included other minerals (the orthorhombic pyroxenes
hypersthene, bronzite and bastite, and the smaragdite variety of
hornblende) which exhibit the same peculiarities of schiller structure;
it is now limited to the monoclinic pyroxenes with this structure. Like
the minerals of similar appearance just mentioned, it is sometimes cut
and polished for ornamental purposes. (L. J. S.)

DIALOGUE, properly the conversation between two or more persons,
reported in writing, a form of literature invented by the Greeks for
purposes of rhetorical entertainment and instruction, and scarcely
modified since the days of its invention. A dialogue is in reality a
little drama without a theatre, and with scarcely any change of scene.
It should be illuminated with those qualities which La Fontaine
applauded in the dialogue of Plato, namely vivacity, fidelity of tone,
and accuracy in the opposition of opinions. It has always been a
favourite with those writers who have something to censure or to impart,
but who love to stand outside the pulpit, and to encourage others to
pursue a train of thought which the author does not seem to do more than
indicate. The dialogue is so spontaneous a mode of expressing and noting
down the undulations of human thought that it almost escapes analysis.
All that is recorded, in any literature, of what pretend to be the
actual words spoken by living or imaginary p