, 'Annalen', bd. x., s. 151.
If we pass from these general considerations to individual examples, we find
that schist is converted, by the vicinity of Plutonic erupted rocks, into a
bluish-black, glistening roofing slate. Here the planes of stratification
are intersected by another system of divisional stratification, almost at
right angles with the former,* and thus indicating an action subsequent to
the alteration.
[footnote] * On the double system of divisional planes, see Elie de
Beaumont, 'Geologie de la France', p. 41; Credner, 'Geognosie Thuringens und
des Harzes', s. 40; and Romer, 'Das Rheinische Uebergangsgebirge', 1844. s.
5 und 9.
The penetration of silica causes the argillaceous schist to be traversed by
quartz, transforming it, in part, into whetstone and silicious schist; the
latter sometimes containing carbon, and being then capable of producing
galvanic effects on the nerves. The highest degree of silicifaction of
schist is that observed in ribbon jasper, a material highly valuable in the
arts,* and which is produced in the Oural Mountains
p 260
by the contact and eruption of augitic porphyry (at Orsk), of dioritic
porphyry (at Aufschkul), or of a mass of hypersthenic rock conglomerated
into spherical masses (at Bogoslowsk).
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