You have got a distinct notion, I hope, of leaf-
crystals; and you see the sort of look they have: you can easily
remember that "folium" is Latin for a leaf, and that the separate
flakes of mica, or any other such stones, are called "folia;" but,
because mica is the most characteristic of these stones, other
things that are like it in structure are called "micas;" thus we
have Uran-mica, which is the green leaf I showed you; and Copper-
mica, which is another like it, made chiefly of copper; and this
foliated iron is called "micaceous iron." You have then these two
great orders, Needle-crystals, made (probably) of grains in rows;
and Leaf-crystals, made (probably) of needles interwoven; now,
lastly, there are crystals of a third order, in heaps, or knots,
or masses, which may be made either of leaves laid one upon
another, or of needles bound like Roman fasces; and mica itself,
when it is well crystallized, puts itself into such masses, as if
to show us how others are made. Here is a brown six-sided crystal,
quite as beautifully chiseled at the sides as any castle tower;
but you see it is entirely built of folia of mica, one laid above
another, which break away the moment I touch the edge with my
knife. Now, here is another hexagonal tower, of just the same size
and color, which I want you to compare with the mica carefully;
but as I cannot wait for you to do it just now, I must tell you
quickly what main differences to look for.
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