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92 Pe - GEOGNOSY.  SEBoox II. 3 
in the same rock, according to the freshness of the surface examined, — 
that they possess but a subordinate value. Nevertheless, when 
cautiously used, colour may be made to afford valuable indications — 
as to the probable nature and composition of rocks. It is in this 
respect always desirable to compare a freshly-broken with a weathered 
piece of the rock.* 
White indicates usually the absence or comparatively small amount 
of the heavy metallic oxides, especially iron. It may either be the 
original colour, as in chalk and cale-sinter, or may ‘be developed by 
weathering, as in the white crust on’ flints and on many porphyries. 
Black may be due either to the presence of carbon (when weathering 
will not change it much), or to some iron-oxide (magnetite chiefly), 
or silicate rich in iron (as hornblende and augite). Many rocks 
(basalts and dolerites particularly) which look quite black on a fresh 
surface, become red, brown, or yellow on exposure, black being com- 
paratively seldom a weathered colour. Yellow, as a dull earth 
colouring matter, almost always indicates the presence of hydrated 
peroxide of iron. In modern volcanic districts it may be due to 
iron-chloride, sulphur, &c. Bright, metallic, gold-like yellow is 
usually that of iron-disulphide. Brown is the normal colour of 
some carbonaceous rocks (lignite), and ferruginous beds (bog-iron- 
ore, clay ironstone, &c.). It very generally, on weathered surfaces, 
points to the oxidation and hydration of minerals containing iron. 
fed, in the vast majority of cases, is due to the presence of granular 
anhydrous peroxide of iron. This mineral gives dark blood-red to 
pale flesh-red tints, As it is liable, however, to hydration, these 
hues are often mixed with the brown and yellow colours of limonite. 
Green, as the prevailing tint of rocks, occurs among schists, when 
its presence is usually due to some of the hydrous magnesian silicates 
(chlorite, tale, serpentine). It appears also among massive rocks, 
especially those of older geological formations, where hornblende, 
olivine, or other silicates have been altered. Among the sedimentary 
rocks it is principally due to ferrous silicate (as in glauconite), 
Carbonate of copper colours some rocks emerald or verdigris green. 
The mottled character so common among many stratified rocks is 
frequently traceable to unequal weathering, some portions of the 
iron being more oxidized than others; while some, on the other 
hand, become deoxidized from the reducing action of decaying 
organic matter. ‘lo the former cause may be attributed the brown 
and yellow hue of the exposed parts of blue clays, to the latter the 
circular green spots so often found among red strata. ; 
Lustre, as an external character of rocks, does not possess the 
value which it has among minerals. In most rocks the oranular 
texture prevents the appearance of any distinct lustre. A completely 
vitreous lustre without a granular texture, is characteristic of yoleanic 
glass. A splendent semi-metallic lustre may often be observed upon 
' Alterations of the colours of minerals and rocks are effected by heat and even by 
sunlight. See Jancttaz, Bull, Soc. Géol. xxix. (1872) p. 300. 

