

12 STRUCTURAL AND FIELD GEOLOGY 
carbon. It is black with a vitreous to submetallic lustre and a black 
streak ; is not easily ignited, and burns almost without smoke or smell. 
Now and again coal (of Carboniferous age) has been converted into 
Graphite (approximately pure carbon), owing to the action of intrusive 
igneous rocks. In such cases it has been subjected to a kind of destruc- 
tive distillation, all its gaseous elements having been eliminated. 
Graphite also occurs in lenticular layers amongst crystalline schists, but 
its precise origin in such conditions is uncertain. Oz/-shale (Bituminous 
schist) is a dark brown or black highly bituminous shale, which is readily 
ignited, but cannot of itself be used as a fuel, owing to its containing so 
much argillaceous matter. Asfhal¢,an admixture of various hydrocarbons, 
is probably in most cases the result of the distillation of coals or other 
organic matter by intrusive igneous masses. It is solid or highly viscous 
at ordinary temperatures ; black or brownish-black, with a pitchy lustre 
and a bituminous odour. It occurs in sheets or layers interstratified 
with sedimentary rocks, or as impregnations in such rocks as limestone 
and sandstone, or occupying fissures and cavities in the same. Not 
infrequently it is met with in veins traversing rocks of various kinds and 
age, and in certain regions it exudes from the ground, forming what are 
known as “tar-springs.” Petroleum is a general name for complex 
hydrocarbons, which are liquid at ordinary temperatures, but vary 
greatly in this respect, some of them being more or less viscous. ‘They 
occur mainly in rocks of a porous character. The origin and source of 
the petroleum or rock-oil derived from the Paleozoic rocks of North 
America is at present unknown. 
Guano is an earthy, white, grey, or yellowish-brown 
accumulation, with a peculiar odour. It consists mainly of 
the excrement of birds, mixed with the offal and débris of 
their repasts, and not infrequently with their own remains, 
together with those of seals, etc. Jt contains some 40 to 50 
per cent. of organic matter and ammonia salts, with 19 to 20 
per cent. or thereabout of phosphate of lime. Immense 
deposits of it have been met with in the rocky islets lying off 
the west coast of South America (Peru) and Africa, but these 
are now largely exhausted. 
Leached Guano.—Guanos long exposed to the action of rain tend to 
have the soluble nitrogenous constituents dissolved out, so that the rela- 
tively non-soluble phosphatic ingredients become in this way concen- 
trated. Some of the phosphoric acid (in the form of ammonia-phosphate) 
is carried down into the underlying rock, which becomes phosphatised. 
Limestones, when these are present, thus tend to become converted into 
tribasic phosphate of lime. 
Rock-guano is a deposit from which all the soluble nitrogenous pro- 
ducts have been leached out, so that only calcium-phosphate remains. 

