THE STRUCT CEE OF ROCK MASSES. 
233 
when sedimentary strata, which have originally contained car- 
bonaceous matter, have undergone metamorphic alteration, it 
not unfrequently happens that the carbon becomes converted 
into graphite, and thus gives rise to graphitic schists, which 
sometimes assume the exact appearance of ordinary mica schist, 
composed of alternate layers of quartz and mica ; the latter 
mineral being in this case represented by the scales of graphite. 
Metallic foliation, i.e. foliated structure developed by the 
occurrence of metallic oxides, sulphides, sulpharsenides, &c., is 
common in many countries, extending at times over consider- 
able areas. Thus- we find in Scandinavia, Brazil, India, and 
elsewhere, ferruginous schists, in which the mica is replaced by 
iron glance or the micaceous form of the sesquioxide of iron. 
The quantity of oxide of iron in these beds of iron schists, as 
they have been termed, sometimes increases so much as to pre- 
ponderate over the stony matter, and form actual beds of iron 
ore often of great magnitude. In like manner we find, in 
several parts of Sweden, schists foliated with zincblende to such 
an extent as to be largely worked for zinc ore, as at Arkersund, 
Shyshyttan, Bovallen, &c. ; whilst at Vena, in Sweden, and 
Alodum, in Norway, similar schists occur (and are worked) con- 
taining cobalt ore and other arsenical compounds. 
Beds of crystalline schists, foliated or impregnated with iron 
and copper pyrites and other sulphides, are common enough in 
many countries, and especially so in Scandinavia, where, from 
the rusty colour assumed by these rocks along the line of out- 
crop, they are called fahlbands. 
With regard to the origin of the crystalline schists, the 
evidence seems in favour of the view that they are merely 
sedimentary beds of sand ; arenaceous, micaceous, and other- 
muds ; and submarine tuffs of eruptive origin, altered by crys- 
tallisation, or what may be termed chemico-molecular action. 
Occasionally an examination under the microscope will reveal 
the contours of the original sand grains, and in some instances, 
as Sorby has shown, the existence of still unobliterated current 
structure, like ripple-drifts for example. The argillaceous 
shales and slates in some parts of Estramadura, in Spain, are 
seen to be converted into a variety of mica schist, when frag- 
ments are found surrounded by or enclosed in the granite. 
Some of the hornblende schists of southern Norway, which con- 
sist almost entirely of crystals of greenish-black hornblende, 
were long ago shown by the author to be the beds of tuff which 
had proceeded from the submarine eruptions of pyroxenite 
(highly augitic trap) in the vicinity, subsequently consolidated 
and re-crystallised in situ. 
When, in addition to the mica and quartz, felspar is also 
present in these rocks, they receive the name of gneiss, or more 
