39 
4. Monoclinic or oblique prismatic system.—The axes may be all equal or 
not, but two of them must be at right angles and one intersecting them 
obliquely. 
5. Triclinic or double oblique prismatic system.— In this system the crystals 
have three unequal axes, which also intersect one another obliquely. 
6. Hexagonal or rhombohedral system.—The crystals belonging to this 
system have four axes, three of which are on the same plane and inclined 
to one another at an angle of 60°, whilst the other axis cuts them at right 
angles in the centre. They are readily distinguished from the foregoing, 
as the number of faces surface planes of a kind around the vertical axis 
are always a multiple of three. 
Crystallography is a very complicated subject, and as minerals in the form of 
crystals are rarely met with by prospectors, it is not worth while entering into it 
more fully. 
The crystalline form in which minerals are found cannot always be depended 
upon, for although a certain mineral always crystallises in the same form, there 
are also casts, called pseudomorphs. The most common of these are the so-called 
“ Devil’s Dice,” which are little cubes of red oxide of iron, being pseudomorphs 
or casts of crystals of iron pyrites, which latter mineral has been decomposed, 
the sulphur being given oil when the iron combined with oxygen, and took the 
form of the cavity. 
METALLIC MINE11ALS. 
THEIR OCCURRENCE AND USE. 
Gold. 
Symbol , Au (Aurum) ; atomic weighty 196'2. 
Gold is always found in the native or metallic state, but generally alloyed with 
silver, copper, or iron, and, although one of the most widely distributed and 
earliest worked metals, it is comparatively rare, owing to the fact that it mostly 
occurs in small particles disseminated through rock or gravel, which renders it 
difficult to extract. It has been always highly prized owing to its beautiful colour, 
the ease with which it can be worked, the‘fact that it does not tarnish when 
exposed to the action of air or water, which, added to its rarity, has caused it to 
be almost universally adopted as the standard of exchange. 
In the early part of the 4th century, many Alchemists spent their lives in seek¬ 
ing what was called the Philosopher’s stone, which was supposed to endow the 
possessor with the power of combining the baser metals in certain proportions, and 
thus transforming them into gold. 
It has now been generally decided by chemists that gold is an element, or in 
other words that it cannot be split up into any more elementary substances, neither 
can it be manufactured from them. In the pure state its specific gravity is very 
high, being 19*3 times as heavy as an equal bulk of water, which physical 
character is taken advantage of in separating it from other minerals, for, except 
platinum and two or three of the very rare metals, it is heavier than any other 
substance. This metal melts at about 2000° Fahrenheit, and can also be volatilised 
at very high temperatures. In the pure state its colour and streak (a mark made by 
it on porcelain) is a deep golden yellow, but in the finely divided state it is either 
red or black, whilst by transmitted light, it is green. 
