bent into a
considerable curve; only if pressed with a knife upon the edge, they
will separate into any number of thinner plates, more and more elastic
and flexible according to their thinness, and these again into others
still finer; there seeming to be no limit to the possible subdivision
but the coarseness of the instrument employed.

[Illustration: FIG. 3.]

Sec. 13. Now, when these crystals or grains, represented by the black spots
and lines in Fig. 3, lie as they do at _a_ in that figure, in all
directions, cast hither and thither among the other materials of the
stone,--sometimes on their faces, sometimes on their sides, sometimes on
their edges,--they give the rock an irregularly granulated appearance
and structure, so that it will break with equal ease in any direction;
but if these crystals lie all one way, with their sides parallel, as at
_b_, they give the rock a striped or slaty look, and it will most
readily break in the direction in which they lie, separating itself into
folia or plates, more or less distinctly according to the quantity of
mica in its mass. In the example Fig. 4, a piece of rock from the top of
Mont Breven, there are very few of them, and the material with which
they are surrounded is so hard and compact that the whole mass breaks
irregularly, like a solid flint, beneath the hammer; but the plates of
mica nevertheless influence the fracture on a large scale, and occasion,
as we shall see hereafter, the peculiar form of the precipice at the
summit of the mountain.[43]

The rocks which are destitute of mica, or in which the mica lies
irregularly, or in which it is altogether absent, I shall call Compact
Crystallines. The rocks in which the mica lies regularly I shall call
Slaty Crystallines.

[Illustration: FIG. 4.]

COMPACT CRYSTALLINES.

Sec. 14. 1st. Compact Crystallines.--Under this head are embraced the large
group of the granites, syenites, and porphyries,--rocks which all agree
in the following particulars:--

Their first characteristic. _Speckledness._

A. Variety of color.--The method of their composition out of different
substances necessitates their being all more or less spotted or dashed
with various colors; there being generally a prevalent ground color,
with other subordinate hues broken over it, forming, for the most part,
tones of silver grey, of warm but subdued red, or purple. Now, there is
in this a very marvellous provision for the beauty of the centr