, 'Annalen', bd. xvi., s. 552, "Strate
of transition argillaceous schist in the Fichtelgebirge, which can be traced
for a length of 16 miles, are transformed into gneiss only at the two
extremities, where they come in contact with granite. We can there follow
the gradual formation of the gneiss, and the development of the mica and of
the feldspathic amygdaloids, in the interior of the argillaceous schist,
which indeed contains in itself almost all the elements of these substances."
Jasper, which,* as I have already remarked, is a production formed by the
volcanic action of augitic porphyry, could only be obtained in small
quantities by the ancients, while another material, very generally and
efficiently used by them in the arts, was granular or saccharoidal marble,
which is likewise to be regarded solely as a sedimentary stratum altered by
terrestrial heat and by proximity with erupted rocks.
[footnote] * Among the works of art which have come down to us from the
ancient Greeks and Romans, we observe that none of any size -- as columns or
large vases -- are formed from jasper; and even at the present day, this
substance, in large masses, is only obtained from the Ural Mountains. The
material worked as jasper from the Rhubarb Mountain (Raveniaga Sopka), in
Altai, is a beautiful ribboned porphyry.
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