Page 81.
"Geometrical limitations."
IT is difficult, without a tedious accuracy, or without full
illustration, to express the complete relations of crystalline
structure, which dispose minerals to take, at different times,
fibrous, massive, or foliated forms; and I am afraid this chapter
will be generally skipped by the reader: yet the arrangement
itself will be found useful, if kept broadly in mind; and the
transitions of state are of the highest interest, if the subject
is entered upon with any earnestness. It would have been vain to
add to the scheme of this little volume any account of the
geometrical forms of crystals an available one, though still far
too difficult and too copious, has been arranged by the Rev. Mr.
Mitchell, for Orr's "Circle of the Sciences;" and, I believe, the
"nets" of crystals, which are therein given to be cut out with
scissors and put prettily together, will be found more conquerable
by young ladies than by other students. They should also, when an
opportunity occurs, be shown, at any public library, the diagram
of the crystallization of quartz referred to poles, at p. 8 of
Cloizaux's "Manuel de Mineralogie;" that they may know what work
is; and what the subject is.
With a view to more careful examination of the nascent states of
silica, I have made no allusion in this volume to the influence of
mere segregation, as connected with the crystalline power. It has
only been recently, during the study of the breccias alluded to in
page 186, that I have fully seen the extent to which this singular
force often modifies rocks in which at first its influence might
hardly have been suspected; many apparent conglomerates being in
reality formed chiefly by segregation, combined with mysterious
brokenly-zoned structures, like those of some malachites. I hope
some day to know more of these and several other mineral phenomena
(especially of those connected with the relative sizes of
crystals), which otherwise I should have endeavored to describe in
this volume.