that method. The facts (and I
can venture to give nothing more than facts) are briefly these:--

Sec. 2. The mineral called mica, described in the course of the last
chapter, is closely connected with another, differing from it in
containing a considerable quantity of magnesia. This associated mineral,
called chlorite, is of a dull greenish color, and opaque, while the mica
is, in thin plates, more or less translucent; and the chlorite is apt to
occur more in the form of a green earth, or green dust, than of finely
divided plates. The original quantity of magnesia in the rock determines
how far the mica shall give place to chlorite; and in the intermediate
conditions of rock we find a black and nearly opaque mica, containing a
good deal of magnesia, together with a chlorite, which at first seems
mixed with small plates of true mica, or is itself formed of minute
plates or spangles, and then, as the quantity of magnesia increases,
assumes its proper form of a dark green earth.

Sec. 3. By this appointment there is obtained a series of materials by
which the appearance of the rock may be varied to almost any extent.
From plates of brilliant white mica half a foot broad, flashing in the
sun like panes of glass, to a minute film of dark green dust hardly
traceable by the eye, an infinite range of conditions is found in the
different groups of rocks; but always under this general law, that, for
the most part, the compact crystallines present the purest and boldest
plates of mica; and the tendency to pass into slaty crystallines is
commonly accompanied by the change of the whiteness of the mica to a
dark or black color, indicating (I believe) the presence of magnesia,
and by the gradual intermingling with it of chloritic earth; or else of
a cognate mineral (differing from chlorite in containing a quantity of
lime) called hornblende.

[Illustration: FIG. 5.]

Such, at least, is eminently the case in the Alps; and in the account I
have to give of their slaty crystallines, it must be understood that in
using the word "mica" generally, I mean the more obscure conditions of
the mineral, associated with chlorite and hornblende.

Sec. 4. Now it is quite easy to understand how, in the compact
crystallines, the various elements of the rock, separating from each
other as they congealed from their fluid state, whether of watery
solution or fiery fusion, might arrange themselves in irregular grains
as at _a_ in Fig. 3, p. 106. Such